Customers often use automation (such as Terrafrom) to manage GCP Projects and their Billing accounts. Currently only a user can be added.
If you try to add service account without any prefix, error appears (from GCP API I believe), saying that the account is service account and needs to be entered with "serviceAccoount:" prefix. If you try to enter the account with the prefix, the UI validation will fail with a message: "'serviceAccount:@cloudbuild.gserviceaccount.com' is not a valid email address".
