I've searched for an answer and there are some old threads stating this can be done when checking in online, but I just went thru the check-in process for my LAX-HEL flight tomorrow and there was no option to enter my KTN. Downloaded the app and there doesn't seem to be a way to re-check-in to repeat the process. I guess I'll have to see if the agent when we drop our bags can do it. Kind of ridiculous that AY participates in TSA-pre and then makes it hard to take advantage of. Or am I missing something?