To open Whisker Menu using a keyboard shortcut, do the following: from the Xfce Settings > Keyboard, on the “Application Shortcuts” tab, click “Add” and use “xfce4-popup-whiskermenu” (without the quotes) for the command. Then assign it a keyboard shortcut (make sure it’s not already in use) and that’s it.

Sumber: https://www.webupd8.org/2013/07/whisker-menu-update-brings-support-for.html