You may do this via www.T-Mobile.com. Login using your registeres T-Mobile ID and password. Once you login it will ask you to verify SSN and account PIN, once verified, on the notification bell that you need to tap will prompt you to set the Pah permission.
Another way is to call in Customer Care at 611 or 1800-937-8997 to get this set up
I assure you, once line permission were all set, we will be able to help you for your@Future request via Messaging and this will also allow you to manage/ view your account@via T-Mobile App or My T-Mobile.