How to use T-Mobile YouTube TV promo if I already subscribe to it?

The real solution is: NO click the link that T-Mobile texts to you on your phone. Email the link to your self and open it on a computer. 

Then go to: Account » Settings » Billing and the option to enter a promo code is right there. 

Enter it and click through.

Its actually very simple and easy if T-Mobile would just let people know how to do it. The lack of support or instruction on their part is utterly baffling!