You'd note it in the Room Request section. If you'd made another request (I think you're the one who had a "fairly specific room request" in another thread) you'd leave the rest of the request blank and just check that box.
But note that checking that "I would rather have my room ready early" will 100% override your other requests no matter how they were made.
So you'll need to decide which is more important - possibly getting your "fairly specific room request" or possibly getting the room early.