This is the code that should work. I used "mySheet" as the worksheet name for the test. Put in the actual name of your worksheet.
Private Sub Workbook_Open()
Where are you putting this code? It should not go in a general module. If you are in Excel, go to icon to left of File, right click and select "View Code". Then in VBE, in upper left dropdown, select Workbook. Then put this code.
old, slow, and confused
but at least I'm inconsistent!