Only requests from URLFetch service will include that header. Otherwise you would have to use the methods listed here [1].
[1] https://cloud.google.com/appengine/docs/standard/python3/authorizing-apps -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/19cc40bd-fcdc-4d47-8996-e354f1ff4fb4%40googlegroups.com.
