Change the Modifier Key

From Musca

Musca can use Mod1, Mod2, Mod3, Mod4, Mod5, Control or Shift as a modifier key via the bind command. The default modifier is Mod4.

Mod4 is commonly bound to X11's Super_L key, which is usually the left Windows key on Linux PCs with a US or UK keyboard layout. If you're on a different system and the default Modifier key does not work, then establish where mod4 (if it exists!) is pointing by using xmodmap:

$ xmodmap | grep mod4
mod4        Super_L (0x7f),  Hyper_L (0x80)

Or run the xev utility, press the left Win key, and watch xev's standard output while you do it. Something like this should appear:

KeyRelease event, serial 27, synthetic NO, window 0xe00002,
   root 0x259, subw 0x0, time 672433, (417,298), root:(418,299),
   state 0x40, keycode 115 (keysym 0xffeb, Super_L), same_screen YES,
   XLookupString gives 0 bytes:
   XFilterEvent returns: False

Using these two tools, find a suitable Modifier key for your system.