Just some added info:
Note that Workbook_Open is a workbook level event and should be placed in
the ThisWorkbook module of the workbook rather than a sheet module or a
general module.
Go into the VBE (alt+F11) and look in the project explorer (Ctrl+R if it
isn't visible). Find your workbook in the "tree" and look for the
ThisWorkbook entry under that. Double click on ThisWorkbook and the
thisworkbook module will open. In the left dropdown at the top select
Workbook and in the right dropdown, select open. It will put in the
skeleton declaration for the Open Event
Private Sub Workbook_Open()
End Sub
put your code in that declaration or call it from that declaration.