You can email your request to:
dcl.reservations@disneycruise.com
Give them your res number(s), party last name(s), and your server's full name, if possible.
DCL will reply to confirm that the request is noted on the reservation, but they will also add that they can't guarantee anything because your server might be in-between contracts when you sail (i.e., on vacation), or they might be working on a different ship, or no longer working for DCL, etc.
One caveat - the only time we requested Ashang but did not get him was when I also included a request for a specific dining rotation. We got the rotation, but did not get Ashang. My guess is that the two were not compatible, and the rotation trumped the server.
On our last cruise, I asked for Ashang and the specific rotation again, but I made it
crystal clear in the email that Ashang trumped the rotation. I know I was playing with fire by making the simultaneous requests again, but this time we didn't get the rotation, but did get Ashang. I am not sure if we just got lucky, or if the careful wording of my e-mail made the difference.