Set alttab to Super_L.

i3/config

@@ -63,7 +63,7 @@ exec --no-startup-id xautolock -time 10 -locker blurlock
 exec_always --no-startup-id ff-theme-util
 exec_always --no-startup-id fix_xcursor
 exec --no-startup-id fcitx -d
-exec --no-startup-id alttab -pk h -nk l -fg "#d58681" -bg "#4a4a4a" -frame "#eb564d" -t 128x150 -i 127x64
+exec --no-startup-id alttab -mk Super_L -pk h -nk l -fg "#d58681" -bg "#4a4a4a" -frame "#eb564d" -t 128x150 -i 127x64
 
 # kill focused window
 bindsym $mod_next+Shift+q kill