Call now and make your request. Rooms are assigned about 4-5 days in advance so you want that in there before you even arrive. You can do online check in 60 days in advance and have options there but only two. I'd pick up the phone and call since your room will be assigned when you check in and if you want to make changes, you will probably have to wait for another room.
I would call. I just did online check in and the only options are picking a floor, near the elevator or attempt to have your room ready when you arrive. There is no way to write something in, a CM has to do that Good Luck!!!
Call and put it on your reservation. If you don't get it, tell the desk CM that you are willing to wait for a room that meets your request and see what they say.