First, super thanks for the very helpful info!
I got the post request to work, using the token of a logged in user to my webapp, but I still can't make it work using the token of a service_account from Cloud Console... any idea?
More details here https://stackoverflow.com/questions/79437731/google-cloud-console-service-account-post-request-to-google-app-script-webapp.
Appreciate any help!