You can call up the reservation line and request a balcony room, and they will add that request to your reservation.
When you get to the resort, mention your request, just to be safe. They will generally try to do whatever they can to get your request, but it all depends on how busy the resort is. If I remember correctly, all the 3rd floor rooms and about half of the 2nd floor rooms at the Poly have a balcony, so it should be a fairly easy request to get!
have a great trip!