Assign Win key + D for Show desktop in XFCE

Remove the initial assignment for the key:

$ xfconf-query --channel xfce4-keyboard-shortcuts --property "/xfwm4/custom/<Super>d" --reset

Now add the shortcut:

$ xfconf-query --channel xfce4-keyboard-shortcuts --property "/xfwm4/custom/<Super>d" --create --type string --set "show_desktop_key"
$ xfconf-query --channel xfce4-keyboard-shortcuts  --list -v | grep -i super