Furthermore of OAuth2 "Service Account" where the Administrator need to grant the acces to the scopes.
Where could I set the "client ID and client secret" of my appengine app on Google Apps Marketplace?
Thanks!
To set up your application ids you need to:
Go to Google console.
Create a project.
In 'Credentials' section you click on Create New Client Idbutton
Recommend watching the following - https://www.youtube.com/watch?v=XxX4oNi_Uyc