You can reserve the show tickets on-line at the RCI website by accessing your reservation, but it is too early. They don't have a schedule finalized for the Allure shows. I wouldn't expect the shows to be open for on-line reservations for several more months.
You'll want to do it on-line too. If you wait and try to book on the ship (which you can do) you might not have as many choices for show times.