Correct, right now that's all you can do. And if you make no other special dining requests, you're very likely to get it.
But if you arrive at port and find that you didn't, once on board, go to wherever they are taking requests for changes to dining assignments (this is mostly people who want main who have second seating) and you may be able to change it, particularly if you have second seating there's a good chance you can change your rotation once on board. No guarantees, but it's always worth asking. (My life motto: "If you don't ask, they can't say 'yes.'" Check your Navigator for the location the day you board.