You can add the KTN during check-in, although their IT seemed to have some glitches. When I entered it on the app, it said the number was invalid. However, web check-in didn't have this problem.

BTW, Flair Airlines do have a policy of charging fees for checking in at the airport. However, their system is so buggy that you just need to mention you weren't able to do it online and they can check you in for free.