I was told at TAP that as a retiree, you have the option of using the clinic with no copays, but that it is not mandatory for any MTF and it is actually discouraged because the length of time that it generally takes to get an appt. If you are Retiree Prime, you should be able to just choose your PCM online like others did above and be good to go. No need to get authorization to go off base. The only stipulation is that you have to live a certain distance from a MTF, but I am not really sure why that is in this day and age of no in-person service centers.
I plan on using prime for health and declining dental. My company has a much better dental plan and it is a boatload less money.