For the vast majority of hotels, room assignment doesn't happen by the person creating the booking. Imagine if Booking.com assigned a guest to Room 401 for instance for next March, it could create chaos as there could be renovation plans or that could be a room reserved for a specific guest that always arrives in March.
Instead, room assignment happens by the hotel (or automatically by the PMS) when the booking is received or closer to the time.
For this reason, the Hotel Charleston mimics actual hotel behaviour. You can create a booking with a requestAreaTypeId which reflects the type of room that you want to book the guest in but not specify the room.
The Hotel Charleston will automatically assign an areaId either when the booking is created or at check-in.