Hi,
after many searches I have figured out how to ‘insert coins’ when starting a Mame game. I have to press Shift-5.
But I don’t want to attach a keyboard to my console, so is there any other way to configure RetroPie for this?
You should just be able to press select to insert coins (on lr-mame4all) but sometimes the hotkey conflicts so you have to change your input_enable_hotkey button to something else if you want select to work in inputting coins.