You call DCL and ask that the request be placed on your reservation. They usually start the assigning process a week or two out, so I'd say before that. If the server has left DCL, is on vacation, or was been moved to another ship or position (such as being moved to Palo), they will just assign you as they would with no request.
It's not a guarantee no matter how or when you do it that you would get that server anyway. Odds are that if the person is on board and that section has room you would, but just keep in mind that there could be any number of reasons why it's not granted.
Edit to add: If you booked through a TA, I believe they have to make the call.
Simple...just call DCL in advance and ask. Odds are great in my experience (if the server is still on board...I guess I didn't need to say that!). We've done it several times with 100% success.
We have tried twice and didn't have our request honored. I know the servers were still on the ship as we saw them on the cruise. We will keep trying. Just know it is not guaranteed.
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