It’s a bit different from airport to airport but CBP has a web page that gives the airport-level details about the hours and position of where to try to do it at each relevant airport. The CBP site is not comprehensive in this regard as at some airports where they do this, they may offer it in ways or at times that go beyond that listed on the website. For example, last quarter, CBP doing MPC at some airports were also willing to do enrollment on arrival or help with getting it done even when the CBP website provided a sort of indication that it wouldn’t do so, at least not at the all the times when CBP had employees doing EOA on the side.