If you're a member of a Trusted Traveler Program, your Known Traveler Number (KTN) — also called your PASS ID — is printed on the back of your membership card. You can also find it by logging into your GOES or TTP account.
To add your KTN to your reservation, go to the Manage Trip section on flyfrontier.com or My Trips in the Frontier app, and update your traveler details.