How to open your Xfce’s application menu using the Windows key in three easy steps:

1. Click the Application menu > “Settings” > “Keyboard”:
   
2. Click the “Application Shortcuts” tab, click the “+ Add” button, enter “xfce4-popup-applicationsmenu” into the “Command” field, and click “OK”:
   
3. When this window appears, press the Windows key (or whatever other shortcut you prefer) and click OK:
   
   Note that the left Windows key will be shown as “Super_L”, this is okay.
4. 参考链接:
5. http://www.clausconrad.com/blog/using-the-windows-key-to-open-xfce-application-menu