Yes, you have to call to do it. If the discount is available for your resort/room type, the CM can easily modify your reservation to reflect the discount. With that said, sometimes you have to be flexible with your resort to take advantage of a discount as not every resort and/or every room type is available with a promo. So unless the YC is definitely where you want to stay regardless of whether or not a discount is offered, I would have a 2nd choice in mind just in case what you have booked isn't available.