Most drop downs are created by showing a new window (or control in .NET) - so you hide it by either calling Hide on it, or Closing it and removing it from the Controls collection of it's "parent" control.
And by the way - instead of Form/Click, you might find that handling the dropdown's LostFocus event (
Control.LostFocus Event (System.Windows.Forms) | Microsoft Docs[
^]) would be a better idea.