PowerShell script to add/remove/list a globally-deployed SPFx extension (with skipFeatureDeployment=true), to selected sites/webs, specifically Application Customizers. Uses OfficeDevPnP PowerShell.

RegisterSPFxExtensionToSelectedSites.ps1

$credentials = Get-Credential

# details of sites
[string[]] $sitesToProcess = 
	"https://chrisobriensp.sharepoint.com/sites/team1",
	"https://chrisobriensp.sharepoint.com/sites/team2", 
	"https://chrisobriensp.sharepoint.com/sites/team3",
	"https://chrisobriensp.sharepoint.com/sites/team4"

# details of custom action/SPFx extension
[guid]$spfxExtension_GlobalHeaderID = "4a79203e-287d-4173-82c9-0111232c6c2e"
[string]$spfxExtName = "COB-SPFx-GlobalHeader"
[string]$spfxExtTitle = "COB-SPFx-GlobalHeader"
[string]$spfxExtGroup = "MyGroup"
[string]$spfxExtDescription = "Adds a global header/footer to the site"
[string]$spfxExtLocation = "ClientSideExtension.ApplicationCustomizer"

function Add-CustomActionForSPFxExt ([string]$url, [Microsoft.SharePoint.Client.ClientContext]$clientContext) {
	Write-Output "-- About to add custom action to: $url"

	# NOTE - using direct CSOM here (rather than Add-PnPCustomAction) for now, due to https://github.com/SharePoint/PnP-PowerShell/issues/1048
	$rootWeb = $clientContext.Web
	$clientContext.ExecuteQuery()
	$customActions = $rootWeb.UserCustomActions
	$clientContext.Load($customActions)
	$clientContext.ExecuteQuery()

	$custAction = $customActions.Add()
	$custAction.Name = $spfxExtName
	$custAction.Title = $spfxExtTitle
	$custAction.Description = $spfxExtDescription
	$custAction.Location = $spfxExtLocation
	$custAction.ClientSideComponentId = $spfxExtension_GlobalHeaderID
	$custAction.Update()
	$clientContext.ExecuteQuery()

	Write-Output "-- Successfully added extension" 	
	
	Write-Output "Processed: $url"
}

function Remove-CustomActionForSPFxExt ([string]$extensionName, [string]$url, [Microsoft.SharePoint.Client.ClientContext]$clientContext) {
	Write-Output "-- About to remove custom action with name '$($extensionName)' from: $url"

	$actionsToRemove = Get-PnPCustomAction -Web $clientContext.Web | Where-Object {$_.Location -eq "ClientSideExtension.ApplicationCustomizer" -and $_.Name -eq $extensionName }
	Write-Output "-- Found $($actionsToRemove.Count) extensions with name $extensionName on this web." 	
	foreach($action in $actionsToRemove)
	{
		Remove-PnPCustomAction -Identity $action.Id
		Write-Output "-- Successfully removed extension $extensionName from web $url." 	
	}

	Write-Output "Processed: $url"
}

# -- end functions --

foreach($site in $sitesToProcess) {
	$authenticated = $false
	$ctx = $null
	try {
		Connect-PnPOnline -Url $site -Credentials $credentials
		Write-Output ""
		Write-Output "Authenticated to: $site"
		$ctx = Get-PnPContext
	}
	catch {
		Write-Error "Failed to authenticate to $site"
		Write-Error $_.Exception
	}

	if ($ctx) {
		# TODO - comment in/out method calls here as you need..
		#Add-PnPCustomActionForSPFxExt $site
		# Remove-CustomActionForSPFxExt $spfxExtName $site $ctx
		Get-PnPCustomAction -Web $ctx.Web | Where-Object {$_.Location -eq "ClientSideExtension.ApplicationCustomizer" }
	}
}