Configure your multimedia keys

Start xev. Move your mouse pointer to the rectangle in it. Type a key. You get an output like this:

KeyPress event, serial 34, synthetic NO, window 0x2c00001,
    root 0x15d, subw 0x2c00002, time 229472, (55,47), root:(1315,731),
    state 0x0, keycode 121 (keysym 0x1008ff12, '''''XF86AudioMute'''''), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 34, synthetic NO, window 0x2c00001,
    root 0x15d, subw 0x2c00002, time 229558, (55,47), root:(1315,731),
    state 0x0, keycode 121 (keysym 0x1008ff12, '''''XF86AudioMute'''''), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

Then open a console and run the command

systemsettings

Select "Shortcuts and Gestures" -> Custom Shortcuts -> Edit -> New -> Global Shortcut -> Command/URL