You just give MS a call and they will add it to your reservation. As of October 14th, you now have to pay for it when you add it to your reservation.
If you prefer not to have to pay for it right now, you can wait and it later, as long as you do it at least 48 hours prior to arrival.
We are thinking of doing this for one night of our August trip but since I will be adding the DxDDP for that one night, I am going to wait until July to do it.