We've played this game many times, many ways. Our TA makes the initial request. I've followed up with an e-mail to
DCL. I've gotten a reply that they can't accept a request for a server due to vacations, etc.....
So, on our last cruise, I asked the head server the best way to do it. He suggested that we do what we've always done--go to Dining Changes after boarding and put in the request. He said that requests received thru Celebration sometimes don't make it to the ship, but if we request on board, they will do everything possible to accomadate us.