In Gnome and XFCE, Alt+Space opens the active window's titlebar menu. This is not only more efficient than a right click on the titlebar but especially useful on a window that doesn't have a titlebar (e.g. a window in full-screen or a sticky note) because then a keyboard shortcut is the only way to access that menu.

I couldn't find a shortcut for it anywhere in KDE's settings.
What's the command for it so I can add it as a custom shortcut?

On my system, this is Alt+F3. It can be configured in SystemSettings->Shortcuts->Global Shortcuts->Kwin->Window Operations Menu.

Ah there it is! I looked in this place but somehow I missed it. Thanks!