Load up a game in lr-mame2016 like "Donkey Kong" for example, and then go into options and "Enable Cheats."
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You then need to RESTART the game (It does not say restart required in the options, but you have to restart) and then when you load up the game again, press TAB on the keyboard to get to the MAME Menu:
Screen Shot 2021-03-02 at 8.02.10 PM.png
And then you can be just like Billy Mitchell and cheat to your hearts desire! :P
Screen Shot 2021-03-02 at 8.02.43 PM.png
I've tested it in quite a few games as so far so good!
Is this correct considering select is usually an 8? It doesn’t make sense to me why the select and button up are the same?
Glad it's working for you.
I'm still learning some of this but just for reference, I think a "-1" would be the equivalent of saying that button is either disabled or undefined. All regular buttons probably should have a number with no sign (+ or -) associated with them. An analog function usually mentions an axis and those can have a negative sign for one direction and a positive sign for the other direction.