I added it to my upcoming trip for December. You have to call WDW, talk to a costumer service rep, and they will add it to your booking.
As a heads up, you will want to know what night you want to do the dessert party. They will ask when making the reservation. The special magic bands will show up as the "color" for both of your magic bands about a day after you make the reservation.
They didn't ask me to pay right away, just added the cost of the package to my booking total.
Hope this helps!