modifier_key = key_lalt
The modifier key is the key you use (in combination with other keys) to trigger shortcut functions in FS-UAE. What the default modifier key is depends on the operating system; Cmd on macOS and Alt on other systems.
key_lsuper(Cmd) on OS X.
key_lalt(Alt) on other operating systems.
This option be can be used to change the default modifier key to almost any other key.