They should have said, but I guessing they didn't because it's easy and indicates it at the checkpoint. You tell the agent you are declining the biometric identification. Do it as soon as you hand the agent your ID, don't step in front of the machine, then follow instruction (probably step alongside so they have a clear view of your face).
You can simply decline by stating to an agent that you do not want your photo taken and want to opt out of a face scan. From there, a TSA agent should follow standard procedure of looking at your ID and your face to verify your identity.
Opting out literally takes zero time. They look at you instead of the computer screen when going through.
On a side note. Amazing how these agencies have zero requirement for transparency. Can we end the era of private companies doing things "for the people" on behalf of the government with no metrics proving they are helping. Cop shot is another prime example of "it works, trust us. "