Adding a Known Traveler Number (KTN) for TSA PreCheck after you have already checked in can be difficult but is often possible. Most airline apps and websites lock the "edit" function for your profile once a boarding pass has been issued. Your best chance is to cancel your check-in on the airline's website, add the KTN to your reservation or profile, and then check in again. If the app won't let you, you must visit the airline's check-in counter at the airport. A gate agent can manually re-issue your boarding pass with the PreCheck indicator. Note that simply having a KTN isn't enough; the indicator (usually a "TSA PRE" logo) must be physically printed on the boarding pass for you to enter the expedited security lane.