I just did this today. The fact that are willing to give me $200 as a convenient trade-in for my 6s that is worth less than that is what pushed me over the edge to get an XR. In effect, $549 for the XR.
You have to complete the trade-in signup at Apple.com, which will show the $200, then do the purchase of the new phone right there online. When the phone is delivered (or picked up) you will be sent a box and shipping label and you ship the phone back to their trade-in partner. Then they will give a $200 credit to the form of payment that you used online to pay the sales tax (and whatever else you purchased, such as Applecare, etc.) This trade-in is an Apple program that has nothing to do with your carrier.
Note: If you choose to pick up the phone at a store do NOT bring your 6s there to trade in -- you won't get the extra $100.