Go to Application > Settings > Keyboard
Click on the central tab “Keyboard shortcuts”
Click on “Add”
in Command, enter “xfce4-popup-whiskermenu”
Now set it to “Start (Windows)” key. “Super L” value might appear.
http://lgallardo.com/en/2009/07/18/accesos-rapidos-en-xfce-4keyboard-shortcuts-in-xfce-4/
Tested and working in Xubuntu 14.04 and 16.04.2