I don't think you can select sleep aboard from the web site booking tool. You have to add it when actually talking to them. It was confusing to me too so I just called to book rather than attempt online. They will give you whatever promo is in place.

Note also that with the sleep aboard, it's not really counted as the first day of your charter. You have to use the first day of your charter to authenticate in some of the web pages and you don't use the sleep aboard day, you use the next day - the official start of your charter.