Depends on what plan you have.

https://www.t-mobile.com/tv-streaming/apple-tv-plus-deal

How do I redeem my Apple TV+ On Us?

T-Mobile postpaid wireless consumers click here to log into your T-Mobile account and add your offer. Once you add your Apple TV+ On Us feature, you will be prompted to activate your subscription with Apple. If someone other than the Primary Account Holder attempts to redeem with Apple, Apple will send the Primary Account Holder a One-Time PIN for verification. If there is no Primary Account Holder designated on the account, the oldest phone number will become the designated phone number.

I also cannot get it to work. It tells me the phone number isn’t authorized even though it shows in my T Mobile account. Any suggestions?

Is the phone number you are using, the Primary Account Holder number?