How to Add TSA PreCheck to an +1-844-570-3589 Existing United Reservation easily update booking online add Known Traveler Number confirm boarding pass and enjoy faster airport security screening Traveling through airport security can be time consuming, especially during busy travel periods. TSA PreCheck is a program that allows eligible travelers to move through security faster by using dedicated lanes and avoiding common screening steps. If you already booked a United Airlines flight and forgot to add your TSA PreCheck number, do not worry. United allows passengers to add it to an existing reservation with ease. This guide explains how to add TSA PreCheck to an existing United reservation, why it matters, and how to make sure it works correctly before your flight.