The idea of a "dummy date" is that you book a cruise you have no intention of being on. You book any cheap cruise and pay a low deposit to lock in your benefits of a reduced deposit, 10% off and an onboard credit. Later, when you find a cruise you actually want to take, you or your
travel agent, modify the reservation and book on the cruise you intend to take.
If you _know_ you want to be on a certain cruise, that's not a "dummy date", it's a true future cruise reservation. Book the stateroom category and number of people you intend to cruise with. Otherwise you run the risk of not being able to add people later.