@acris I tried with a keyboard but it isn't working inside the ROM, only in the menu. The only functions that are working using my 8 button game pad are:
Hotkey + Start = Exit the game
Hotkey + Y = Opens a window to choose a slot to save, but didn't show any slot option.
Hotkey + X = Opens a window to choose a slot to load, w/ the same problem of the saving.
Hotkey + A = Reset the game
I'm trying it in Mortal Kombat I (Mame version).

@acris I tried with a keyboard but it isn't working inside the ROM, only in the menu. The only functions that are working using my 8 button game pad are:
Hotkey + Start = Exit the game
Hotkey + Y = Opens a window to choose a slot to save, but didn't show any slot option.
Hotkey + X = Opens a window to choose a slot to load, w/ the same problem of the saving.
Hotkey + A = Reset the game
I'm trying it in Mortal Kombat I (Mame version).

I have the same problem. I use an arcade control to operate Recalbox, most of the emulators the access to it's menus is pressing Hotkey (in my case "Selecet) + B. My P1 control have 8 buttons, like a SNES control. B, A, X, Y, R, L, SELECT, START. How to set this "TAB" or "R2" button different?