to make sure the responses are clear on that....
you must program the specific key to the mpem, think of it like a car key, it will fit your ford, not everybody's ford, but you can have 3 identical keys to fit your ford.
you cannot program the mpem to not care which key is plugged in, or "any key"
but you can program multiple key's to the specific mpem, or even say 2 mpem's to the same key code, and have multiple keys that interchange.