Ubuntu – 17.10 Super key to open applications

I had the same issue and I finally resolved it. Since there is no way to solve this with Xfce alone, one needs the small tool ksuperkey. It was originally developed for KDE, but it works fine on other desktops. You basically install ksuperkey or compile it yourself, and bind your menu to "alt+f1". Once you run ksuperkey, the superkey will behave as one would expect. I highly recommend to put this tool into some form of autostart, so that ksuperkey gets started on startup. Set it and forget it!

What's your screen's resolution? The keyboard shortcut list is not displayed on small monitors.