A while ago I published some other functions for Chat and messaging, here is one for Pushover

Send-Pushover.ps1

#region License

<#
	{
		"info": {
			"Statement": "Code is poetry",
			"Author": "Joerg Hochwald",
			"Contact": "joerg.hochwald@outlook.com",
			"Link": "http://hochwald.net",
			"Support": "https://github.com/jhochwald/MyPowerShellStuff/issues"
		},
		"Copyright": "(c) 2012-2015 by Joerg Hochwald. All rights reserved."
	}

	Redistribution and use in source and binary forms, with or without modification,
	are permitted provided that the following conditions are met:

	1. Redistributions of source code must retain the above copyright notice, this list of
	   conditions and the following disclaimer.

	2. Redistributions in binary form must reproduce the above copyright notice,
	   this list of conditions and the following disclaimer in the documentation and/or
	   other materials provided with the distribution.

	3. Neither the name of the copyright holder nor the names of its contributors may
	   be used to endorse or promote products derived from this software without
	   specific prior written permission.

	THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
	IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
	AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
	CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
	SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
	THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
	OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	POSSIBILITY OF SUCH DAMAGE.

	By using the Software, you agree to the License, Terms and Conditions above!
#>

#endregion License

function Send-Pushover {
<#
	.SYNOPSIS
		Sends a push message via Pushover
	
	.DESCRIPTION
		We established a lot of automated messaging and push services, Pushover was missing!
		We do not use Pushover that much, but sometimes it is just nice to have the function ready...

	.EXAMPLE
		PS C:\> Send-Pushover -User "USERTOKEN" -token "APPTOKEN" -Message "Test"

		Send the message "Test" to all your devices. The App Name is displayed a title of the push

	.EXAMPLE
		PS C:\> Send-Pushover -User "USERTOKEN" -token "APPTOKEN" -Message "Test" -device "Josh-iPadPro"

		Send the message "Test" to the device with the name "Josh-iPadPro". The App Name is displayed a title of the push

	.EXAMPLE
		PS C:\> Send-Pushover -User "USERTOKEN" -token "APPTOKEN" -Message "Test" -title "Hello!" -sound "cosmic"

		Send the message "Test" to all your devices. It will have the Title "Hello!" and use the notification sound "cosmic"

	.EXAMPLE
		PS C:\> Send-Pushover -User "USERTOKEN" -token "APPTOKEN" -Message "Nice URL for you" -title "Hello!" -url "http://hochwald.net" -url_title "My Site"

		Send the message "Nice URL for you" with the title "Hello!" to all your devices. The Push conains a link to "http://hochwald.net" with the URL title "My Site"

	.PARAMETER User
		the user/group key (not e-mail address) of your user (or you), viewable when logged into our Pushover dashboard
	
	.PARAMETER Message
		Your message, can be HTML like formated
	
	.PARAMETER token
		Your Pushover application API token
	
	.PARAMETER device
		Your device name to send the message directly to that device, rather than all of the devices (multiple devices may be separated by a comma)
	
	.PARAMETER title
		Your message title, otherwise your app name is used
	
	.PARAMETER url
		A supplementary URL to show with your message
	
	.PARAMETER url_title
		A title for your supplementary URL, otherwise just the URL is shown
	
	.PARAMETER priority
		The Push priority (-2 to +2)
	
	.PARAMETER sound
		The name of one of the sounds supported by device clients to override the user's default sound choice
	
	.NOTES
		Based on our Send-SlackChat function
	
	.LINK
		Info: https://pushover.net
	
	.LINK
		API: https://pushover.net/api
	
	.LINK
		Send-SlackChat
#>
	
	[CmdletBinding(ConfirmImpact = 'None',
				   SupportsShouldProcess = $true)]
	param
	(
		[Parameter(Mandatory = $true,
				   Position = 0,
				   HelpMessage = 'the user/group key (not e-mail address) of your user (or you), viewable when logged into our Pushover dashboard')]
		[ValidateNotNullOrEmpty()]
		[System.String]$User,
		[Parameter(Mandatory = $true,
				   Position = 1,
				   HelpMessage = 'Your message, can be HTML like formated')]
		[ValidateNotNullOrEmpty()]
		[System.String]$Message,
		[Parameter(Mandatory = $false,
				   Position = 2,
				   HelpMessage = 'Your Pushover application API token')]
		[ValidateNotNullOrEmpty()]
		[System.String]$token,
		[Parameter(HelpMessage = 'Your device name to send the message directly to that device, rather than all of the devices (multiple devices may be separated by a comma)')]
		$device,
		[Parameter(HelpMessage = 'Your message title, otherwise your app name is used')]
		$title,
		[Parameter(HelpMessage = 'A supplementary URL to show with your message')]
		$url,
		[Parameter(HelpMessage = 'A title for your supplementary URL, otherwise just the URL is shown')]
		$url_title,
		[Parameter(HelpMessage = 'The Push priority (-2 to +2)')]
		[ValidateSet('-2', '-1', '0', '1', '2')]
		$priority = "0",
		[Parameter(HelpMessage = 'The name of one of the sounds supported by device clients to override the user''s default sound choice')]
		[ValidateSet('pushover', 'bike', 'bugle', 'cashregister', 'classical', 'cosmic', 'falling', 'gamelan', 'incoming', 'intermission', 'magic', 'mechanical', 'pianobar', 'siren', 'spacealarm', 'tugboat', 'alien', 'climb', 'persistent', 'echo', 'updown', 'none')]
		$sound = "pushover"
	)
	
	BEGIN {
		# Cleanup all variables...
		Remove-Variable -Name "uri" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
		Remove-Variable -Name "body" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
		Remove-Variable -Name "myBody" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
		Remove-Variable -Name "myMethod" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
	}
	
	PROCESS {
		Set-Variable -Name "uri" -Value $("https://api.pushover.net/1/messages.json")
		
		# Build the body as per https://pushover.net/faq#library
		# We convert this to JSON then...
		Set-Variable -Name "body" -Value $(@{
			token = $token
			user = $User
			message = $Message
		})
		
		# Sent a push to a special Device? Could be a list seperated by comma
		if ($device) {
			$TmpBody = @{ device = $device }
			$body = $body + $TmpBody
			$TmpBody = $null
		}
		
		# Want a Title for this Push?
		if ($title) {
			$TmpBody = @{ title = $title }
			$body = $body + $TmpBody
			$TmpBody = $null
		}
		
		# Attach a URL to the push?
		if ($url) {
			# Encode the URL if possible
			if ((Get-Command ConvertTo-UrlEncoded -errorAction SilentlyContinue)) {
				try {
					$url = (ConvertTo-UrlEncoded $url)
				} catch {
					# Argh! Use a unencoded URL
					$UrlEncoded = ($url)
				}
			} else {
				# Use a unencoded URL
				$UrlEncoded = ($url)
			}
			$TmpBody = @{ url = $UrlEncoded }
			$body = $body + $TmpBody
			$TmpBody = $null
		}
		
		# Give the URL a nice title. Just URLs suck!
		if ($url_title) {
			$TmpBody = @{ url_title = $url_title }
			$body = $body + $TmpBody
			$TmpBody = $null
		}
		
		# Set a Priotity for this push
		if ($priority) {
			$TmpBody = @{ priority = $priority }
			$body = $body + $TmpBody
			$TmpBody = $null
		}
		
		# Special Sound?
		if ($sound) {
			$TmpBody = @{ sound = $sound }
			$body = $body + $TmpBody
			$TmpBody = $null
		}
		
		# Convert the Body Variable to JSON Check if the Server understands Compression,
		# could reduce bandwidth Be careful with the Depth Parameter, bigger values means less performance
		Set-Variable -Name "myBody" -Value $(ConvertTo-Json $body -Depth 2 -Compress:$false)
		
		# Method to use for the RESTful Call
		Set-Variable -Name "myMethod" -Value $("POST" -as ([System.String] -as [type]))
		
		# Use the API via RESTful call
		try {
			(Invoke-RestMethod -Uri $uri -Method $myMethod -Body $body -UserAgent "Mozilla/5.0 (Windows NT; Windows NT 6.1; en-US) NET-Experts WindowsPowerShell Service $CoreVersion" -ErrorAction Stop -WarningAction SilentlyContinue)
		} catch [System.Exception] {
			<#
				Argh!
				That was an Exception...
			#>
			
			# Convert the Exception to a usable String
			$exceptionMessage = $_.Exception.ToString()
			
			# Show a warning (No error) with the Infos...
			Write-Warning -message "Caught exception $exceptionMessage during execution of URI $uri"
			Write-Warning -message "Could not send notification to your Slack $User"
			Remove-Variable -Name "exceptionMessage" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
		} catch {
			# Whoopsie!
			# That should not happen...
			Write-Warning -message "Could not send notification to your Slack $User"
		} finally {
			# Cleanup all variables...
			Remove-Variable -Name "uri" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
			Remove-Variable -Name "body" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
			Remove-Variable -Name "myBody" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
			Remove-Variable -Name "myMethod" -Force -Confirm:$false -ErrorAction:SilentlyContinue -WarningAction:SilentlyContinue
		}
	}
	
	END {
		# Do a garbage collection
		if ((Get-Command invoke-gc -errorAction SilentlyContinue)) {
			invoke-gc
		}
	}
}