I used the answer on Have application menu open up with only Windows key to open the applications dock with Win key (works perfectly). Is there a way to have it close again using Win key? It just stays there unless I manually click somewhere else on the screen.
Please, use my updated answer:
gsettings set org.gnome.mutter overlay-key "'Super_L'" gsettings set org.pantheon.desktop.gala.behavior overlay-action "'wingpanel --toggle-indicator=app-launcher'"
This solution works fine both on open and close Slingshot (the application launcher).
What you ask is not possible if not by hacking the source code of Slingshot (the application launcher).
After Slingshot has been opened with the Win key, it gets the focus. So you would have to be able to "tell it" that it should close if you press the Win key again.