Check out the rates for a 3rd or 4th person, that might give them a clue that two beds are needed. Otherwise, a comment on the rez followed by a call to the property may be the only way to get what you want. So many companies try to second-guess what the customer really wants and fails to listen to what they say. Generic forms don't always fit the needs.

I will book a double if the rate is lower and put comments saying a King is ok. Most times I get a king, but can't complain (well, don't complain) if I end up with a double room.