How do I choose a seat on Allegiant Air? Visit the official Allegiant Air website first. Second, log in to your account using the proper username and password. Then, in the manage booking section, enter your flight booking number.Additionally, you must also input the passenger's last name. You will then get a list of your reservations. Allegiant Air Seat Selection Policy requires you to select the flight for which you must select a seat. Click on the tab for seat selection now. You will also be sent to a new website where a seat map can be found. Decide on a seat based on your preferences. You must now confirm and pay for the Allegiant Air seat selection. Finally, the airline will send you a confirmation email after the payment procedure is finished.