You need to parse the JWT to get the user_id
The user_id is only populated if the user has chosen to share their user ID with the extension and you have identity enabled
The user’s Twitch user ID. This is provided only for users who allow your extension to identify them. There are no guarantees this will be available, as users can revoke an extension’s ability to identify them at any time. Users who revoke an extension’s ability to identify them are issued a new opaque identifier.