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

magenta10119565 wrote:

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!

This is the correct answer. Copy the promo code you receive from T-mobile and paste it in promo code field in YouTube TV: Account → Settings → Billing. You will get confirmation that it applied and will start with your next billing cycle.