Firebase FCM - how to test Retry-After with exponential back-off

Question

I want to implement the recommended exponential back-off trying to honour the Retry-After header eventually returned by the Firebase Cloud Messaging, using HTTP 1.1 with PHP and cURL.

My problem (which is actually a good thing so far) is that I'm not getting (yet) any Retry-After header becasue all notifications get delivered.

Is there a way to simulate a rejected message like when receiving a Retry-After header, so to test if my code will work correctly in such scenario?

For example by setting a property in the payload or header when sending the notification?

Thanks!