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) {
  let url = ScriptApp.getService().getUrl();  // Ends in /exec (for a web app)
  url = `${
      url.slice(0, -4)}usercallback?state=`;  // Change /exec to /usercallback
  const stateToken = ScriptApp.newStateToken()
                         .withMethod(callbackFunction)
                         .withTimeout(120)
                         .createToken();
  return url + stateToken;
}