How do I open the application launcher ("start menu") of Kubuntu with my Windows key? Similar to how the Start menu in Windows is opened. I don't want to use Alt+F1.
About modifier keys
Modifier keys like Ctrl, Alt and also the Meta (also called "Super" or "Windows" key, are meant to operate only as modifier keys. This design works its way down to the roots of all the libraries under the GUI you see using KDE: Qt, Xorg, etc.
Your exact question is here on the KDE Forums by the way: Super key to load Application Menu.
It seems however, that there's a large audience for this feature. So, in case you really want this to work, you could try the
For easy installation, install the package available as a download at the bottom of the page.
Now let's hope this will be available in Ubuntu soon. Feel free to create a bug report (feature request) for Ubuntu to get this packaged up!