There are several locations you can do it, once you're at Walt Disney World. It can be done at Guest Services at your resort. (This might be labeled the Concierge Desk - as it was at Pop Century when I was there at the beginning of the month)
It can be done at Guest Relations. There is a location at the entrance of each of the four theme parks, at the International Gateway at Epcot, between Team Mickey and Arribas Brothers at the Downtown Disney Marketplace, and next to Wetzel's Pretzels at the Downtown Disney West Side.