Or, use: Windows(1).WindowState = xlMinimized
in either ThisWorkbook in the automatically-open part of the code,
or the userform code.

You may need to change the "Windows(1)" part to fit your situation,
since the window  a user may be using may not be the first one
running on Excel.

And, remember to open it again with something like: Windows
(1).WindowState = xlMaximized if you need the spreadsheet  needs to
be viewed later.