Add custom user action through Powershell CSOM

CustomUserAction.ps1

Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll" 
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Publishing.dll"

# Authenticate with the SharePoint site. 
# 
$siteUrl = Read-Host -Prompt "Enter web url:" 
$username = Read-Host -Prompt "Enter Username:" 
$password = Read-Host -Prompt "Enter password" -AsSecureString 
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteUrl) 

# SharePoint Online
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password) 
$ctx.Credentials = $credentials

$rootWeb = $ctx.Web
$ctx.Load($rootWeb) 
$caColl = $rootWeb.get_userCustomActions()
$ctx.Load($caColl) 
$ctx.ExecuteQuery() 

$caColl | ForEach-Object {
    if ($_.title -eq 'Invoke GetItemsCount Action') { 
    $_.DeleteObject()
    Write-Host 'Existing item deleted.'
    }
}

$cUIExtn = "<CommandUIExtension><CommandUIDefinitions><CommandUIDefinition Location=""Ribbon.List.Share.Controls._children"">
<Button Id=""Ribbon.List.Share.GetItemsCountButton"" Alt=""Get list items count"" Sequence=""11"" Command=""Invoke_GetItemsCountButtonRequest"" LabelText=""Get Items Count"" TemplateAlias=""o1"" Image32by32=""_layouts/15/images/placeholder32x32.png"" Image16by16=""_layouts/15/images/placeholder16x16.png"" />
</CommandUIDefinition></CommandUIDefinitions><CommandUIHandlers>
<CommandUIHandler Command=""Invoke_GetItemsCountButtonRequest"" CommandAction=""javascript: alert('Total items in this list: '+ ctx.TotalListItems);""          
EnabledScript=""javascript: function checkEnable() { return (true);} checkEnable();""/></CommandUIHandlers></CommandUIExtension>"

$newUCA = $caColl.Add()
$newUCA.set_registrationId("100");
$newUCA.set_registrationType("List");
$newUCA.set_location('CommandUI.Ribbon');
$newUCA.set_title('Invoke GetItemsCount Action');
$newUCA.set_commandUIExtension($cUIExtn)

$newUCA.update()
$ctx.ExecuteQuery() 

Write-Host 'Custom action added.'