Xfce has a keyboard-shortcuts editor. If the memu is already listed, you can change the default keystroke. If it isn't, add an entry for xfce4-popup-applicationsmenu.
The usual keystroke for a Linux menu is Alt-F1. If you don't like that, I'd suggest not using the Super (aka Windows) key alone: it's convenient to use it as a modifier with another key to launch a range of programs, to save using the menu. I have Super-a for accounts, Super-c for calculator, Super-e for editor, and so on.