Here are the steps:
Participation is optional and requires an explicit opt-in to use biometric data for identity verification. Guests who are eligible and want to opt-in must sign up using their Mileage Plan account. Once your passport number and KTN are listed in the travel documents section of your profile,
you can opt-in to the TSA PreCheck® Touchless ID program.
Once you have opted in, an updated Touchless Identity icon will be added to your electronic boarding pass whenever you are traveling through an eligible airport. Airport security line monitors will look for this icon to allow you to enter the dedicated Touchless ID line. Look for the signage to the TSA PreCheck Touchless ID® security checkpoint at participating airports.
Guests may opt out at any time.
So make sure you have your KTN and Passport number in your account...