Not sure how to "call or email the concierge staff" - I emailed our Dreams Unlimited cruise specialist and she said that we have to wait until we are on board and go to the activities desk that handles the cabanas. I am thinking this is a risk and they could all be gone by then ...
Your travel agent should know more about your trip than you do! If you are concierge she should a either told you to call concierge, or booked it for you.