In the LH app, go to my bookings then select View Booking. Scroll down to passenger info. You should see section labelled Frequent Flyer card. If you look at that it should contain your Mileageplus number. There should also be a section called Visa and Immigration info. Click on that, and then as part of the info there should be a checkbox that says Person has a redress code or known traveller number. Check that and make sure your Known Traveler Number is in the given box.