$computer = gc env:computername
$key = "XXXXX-XXXXX-XXXXX-XXXXX-XXXXX"
$service = get-wmiObject -query "select * from SoftwareLicensingService" -computername $computer
$service.InstallProductKey($key)
$service.RefreshLicenseStatus()
Extraído do link . Isso só funcionou para mim ao ativar máquinas há alguns dias atrás