Obtain an id token in the Gmail add-on for a backend service authentication
问题 The background I'm using the Google Apps Script to create a Gmail Add-on. Via this plugin, I would like to connect to my backend server (a non-Google service) using a REST service request. The request has to be authorised. When authorised, I could then make requests to that server to receive data associated with that user in the database. I'm already using Google sign-in in my webapp to sign in to the backend service - at the front end, I receive the id_token inside of the GoogleUser object