T-Mobile app steps
 
Make sure the T-Mobile app is on the latest version before attempting these steps.

Download and log in to the T-Mobile app .
Select Account or More.
Select Profile Settings.
Select Line Settings.
Select Request a transfer PIN.
Customer will be directed to verify their identity
Have the customer select a phone number to receive a text OTP
Select Continue
Customer will need to enter 6-digit OTP 
Select Continue
Select Get a transfer PIN
Inform the customer to provide that PIN to their NSP