Class StateTokenBuilder

// Reusable function to generate a callback URL, assuming the script has been published as a
// web app (necessary to obtain the URL programmatically). If the script has not been published
// as a web app, set `var url` in the first line to the URL of your script project (which
// cannot be obtained programmatically).
function getCallbackURL(callbackFunction){
  var url = ScriptApp.getService().getUrl();      // Ends in /exec (for a web app)
  url = url.slice(0, -4) + 'usercallback?state='; // Change /exec to /usercallback
  var stateToken = ScriptApp.newStateToken()
      .withMethod(callbackFunction)
      .withTimeout(120)
      .createToken();
  return url + stateToken;
}