I bet this is a hint in the right direction, but I'm not sure what to do with this information just yet.

This works. I could execute this as a logon script or WEM action. But I haven't found an IF clause yet, to determine, if this has to be run. I don't want to runs this every time without a need for it.

@jvitech That might work, but I would rather do a query. Will continue research today.

Is there any way in Registering the Microsoft.ScreenSketch AppX, without knowing the Full Path to the AppxManifest.xml? Because there seems to be no way for the user (without elevated permissions) to research that path. If you know the path beforehand, it's "easy" ™️ . But just by knowing the app is missing, but not knowing the path from the user session, seems to be complicated. I could use the very same trick I already use for Teams2: During the Packer build I query the path, which is easy, with elevated permissions, and then same it as a machine environment variable. I can then reference the machine variable as a user login script, like this:

if (!(Get-AppxPackage -Name "MSTeams")) {
    Write-Host "Teams v2 will be registered!" -ForegroundColor Green
    Add-AppXPackage -Register -DisableDevelopmentMode "$ENV:TeamsVersionPath\AppXManifest.xml"
}

I tried the wrong approach, this seems to work, will now test via WEM:

if (!(Get-AppxPackage -Name Microsoft.ScreenSketch)) {
    Add-AppxPackage -MainPackage "Microsoft.ScreenSketch_8wekyb3d8bbwe" -RegisterByFamilyName
}