I think the thread replies so far are missing the point of the issue...

Or maybe I am. But I think and @warawara correct me if I am wrong, that you want to have a different button then the select act as the hotkey button.

That or you do NOT want to select button to add coins.

I have done both in my builds. I like the coin acceptor for both the retro feel and to help limit play with myself and my kid.

Not to say it is the best solution but what I have done;
I wired the coin acceptor as a button and in the retropie I set it to "Left trigger" (as I don't use that in any of my games)
I then globally configure the emulators I use to use the left trigger instead of select for coin. It addresses 99% of the coin arcade games and there are a few custom deals that you have to work with. Player specific coin slot games are an example such as Simpsons where each player has a coin slot and it determines the character you play. For that I added a toggle switch that lets me "shift" the coin acceptor to the player I want to activate. But like I said, in most cases I'm fine leaving it on player 1.

Select in this method is just select and a hot key. No coins are added when pressed. This is basically the same solution as @DougA wrote with a few minor changes in what button the coin acceptor is assigned to.

For the other situation (IE independent hot key);
I had a small momentary puss button that I added to a build as a button for player 1. So player 1 had 9 buttons; 6 action, start select and this extra button. In the emulation station input configuration I set the extra button as the hot key. I only pressed that extra button when I got to the last entry of setting a hotkey. So instead of pressing select and start to exit you would press what I labeled the FN button and start. The same applies for all the other functions at that point as well. FN + B = reset, FN + X = menu and so on.

If we are way off base in what we are explaining, you will need to re-explain what you want to happen in your set up.

Any idea why my Select button suddenly stop working as my coin button in MAME?