I was going to order a cake for DW's birthday during one of our previous trips. I aksed about it when I booked our ADR for the meal and the advice I was given was to phone the restaurant a couple of days before and arrange everything directly.
In the end I ordered one via a birthday Wishes cruise which ended up getting cancelled due to a huge thunderstorm
I suspect the advice I was given to phone the restaurant a couple of days before is still good though
