My microwave doesn't have a popcorn button (and in fact, I never saw a microwave with a "popcorn" button. Must be an American thing.)
Neither does mine (and I think you may very well be right there).
It's been ages since I made micro-popcorn so I don't remember how I did them... but the first time ever I popped in the micro I timed it.
It was my idea to program one of the buttons to be my popcorn button, but I didn't even need to do that as the time I found perfect for one bag (only one size available here), was a time I could reach by pressing the START-button a couple of times.
The defrost and roast chicken
-button, however, is fantastic! -works brilliantly every time!
(I should mention that my oven is a combo of grill, convection, and micro-oven).