How can I use the Windows key as global hotkey for Greenshot?

There are some special keys that are handled differently by Windows and therefore cannot be inserted into the hotkey configuration panel (Settings > Hotkeys) directly. E.g. this is the case with the Windows or Pause keys. However, you can still use them with Greenshot - you just have to edit Greenshot's configuration file manually in order to do so:

See also:
Where does Greenshot store its configuration settings?