You can choose your seats at check-in, or you can do it beforehand using the 6-letter Sun Country confirmation code that the operator will provide.

1. Visit Allegiant Air's official website or call (701) 505-2188.

2. The "manage my travel/check-in" option can be found by selecting the "travel" menu.

3. You will be taken to the "Welcome to My Trips" page if you select that option.

4. By accessing via confirmation, enter all the check-in information, including first and last names and confirmation for Allegiant Air seat selection.

5. Select "Find My Trip." You can find all the information about the reserved seat on the following page.

6. To access the seat selection option, click the following tab. Tap that choice after that.

7. Choose your seat and complete the Allegiant Airlines check-in procedures.

Source:- https://www.jointium.com/read-blog/23740