Check if a given Name is availible as Office365/Azure Tenant Name and optional return the Tenant ID if the Tenant exists.

Get-MicrosoftCloudTenantInfo.ps1

function Get-MicrosoftCloudTenantInfo
{
	<#
			.SYNOPSIS
			Check if a given Name is availible as Office365/Azure Tenant Name
	
			.DESCRIPTION
			Check if a given Name is availible as Office365/Azure Tenant Name and optional return the Tenant ID if the Tenant exists.
	
			.PARAMETER name
			Check if a given Name is availible as Office365/Azure Tenant Name
	
			.PARAMETER id
			Get the Tenant ID
	
			.EXAMPLE
			PS C:\> Get-MicrosoftCloudTenantInfo -name 'Contoso' -id
			The Tenant ID of contoso.onmicrosoft.com (contoso) is XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX

			.EXAMPLE
			PS C:\> Get-MicrosoftCloudTenantInfo -name 'Contoso'
			The Tenant contoso.onmicrosoft.com (contoso) is available

			.EXAMPLE
			PS C:\> Get-MicrosoftCloudTenantInfo -name 'Contoso'
			WARNING: The Tenant contoso.onmicrosoft.com (contoso) is taken!
	
			.NOTES
			Author: Joerg Hochwald (http://jhochwald.com)
			Copyright: 2018 Joerg Hochwald. All rights reserved.
			License: BSD 3-Clause "New" or "Revised" License
			Version: 1.0.0
			Changelog: Initial Public Release
	#>
	
	param
	(
		[Parameter(Mandatory,
				ValueFromPipeline,
				Position = 1,
		HelpMessage = 'Check if a given Name is availible as Office365/Azure Tenant Name')]
		[ValidateNotNullOrEmpty()]
		[string]
		$name,
		[Alias('TenantID')]
		[switch]
		$id
	)
	
	Begin
	{
		# Define some defaults
		$ST = 'Stop'
		$SC = 'SilentlyContinue'

		# Do not use SSLv3 for any kind of Web Requests
		If ([Net.ServicePointManager]::SecurityProtocol -notmatch 'TLS12')
		{
			[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::TLS11
			[Net.ServicePointManager]::SecurityProtocol += [Net.SecurityProtocolType]::TLS12
		}

		# Cleanup
		$available = $null
		$TenantID = $null
		$TenantLongName = $null

		# Make the tenant name lowercase all the way, just in case!
		$name = $name.ToLower()

		# Where to check
		$uri = 'https://portal.office.com/Signup/CheckDomainAvailability.ajax'

		# OK, the Body looks creapy
		$body = 'p0=' + $name + '&assembly=BOX.Admin.UI%2C+Version%3D16.0.0.0%2C+Culture%3Dneutral%2C+PublicKeyToken%3Dnull&class=Microsoft.Online.BOX.Signup.UI.SignupServerCalls'
	}

	Process
	{
		# get the Info via Rest
		$paramInvokeRestMethod = $null
		$paramInvokeRestMethod = @{
			Method        = 'Post'
			Uri           = $uri
			Body          = $body
			ErrorAction   = $SC
			WarningAction = $SC
		}
		$response = (Invoke-RestMethod @paramInvokeRestMethod)

		# Error handler
		$valid = $response.Contains('SessionValid')

		if ($valid -eq $false)
		{
			# Whoops
			Write-Error -Message $response -ErrorAction $ST
			Exit
		}

		# Looks good
		$available = $response.Contains('<![CDATA[1]]>')
	}

	End
	{
		# Internal log Name
		$TenantLongName = $name + '.onmicrosoft.com'

		if ($available) 
		{
			Write-Output -InputObject ('The Tenant {0} ({1}) is available' -f $TenantLongName, $name)
		}
		else 
		{
			if ($id) 
			{
				# Cleanup
				$TenantID = $null

				try 
				{
					# Build the UIR
					$TenantIDURI = 'https://login.windows.net/' + $name + '.onmicrosoft.com/.well-known/openid-configuration'
					
					# Get the Info via regular call and split it
					$paramInvokeWebRequest = $null
					$paramInvokeWebRequest = @{
						Uri         = $TenantIDURI
						ErrorAction = $ST
					}
							
					$TenantID = ((Invoke-WebRequest @paramInvokeWebRequest | ConvertFrom-Json -ErrorAction $ST).token_endpoint.Split('/')[3])
					
					Write-Output -InputObject ('The Tenant ID of {0} ({1}) is {2}' -f $TenantLongName, $name, $TenantID)
				}
				catch 
				{
					# Whoops
					Write-Warning -Message 'The Tenant is taken, but we where unable to get the Tenant ID!!!'
				}
			}
			else 
			{
				Write-Warning -Message ('The Tenant {0} ({1}) is taken!' -f $TenantLongName, $name)
			}
		}

		# Cleanup
		$available = $null
		$TenantID = $null
		$TenantLongName = $null
	}
}

Hello!
https://portal.office.com/Signup/CheckDomainAvailability.ajax does not work and returns error, is there another endpoint that we can use?

Hello!
https://portal.office.com/Signup/CheckDomainAvailability.ajax does not work and returns error, is there another endpoint that we can use?
Not that I know. At the Moment, I use this web Site: https://o365.rocks