I think you'll need to:
a) order direct from t-mobile
b) specify that you are trading in at the time you order
I just traded in a 6+ as part of buying an iPhone 8. Never again. The big print says $300, the T&Cs (not even posted on the website), say that the device will be evaluated and you will receive up to $300. I processed the order via a phone call and later found that they estimated my phone's value at $109 without even asking me any questions, or telling me their valuation.
Oh, and it can take up to 3 months to begin the monthly discounts that constitute payment for the trade-in.