Workbook Open in VBA

Hello Excel friends,

I have the following in a module to make it automatically open Workbook B when I open Workbook A. It doesn't automatically open B. When I go to the module and click the RUN button, it opens B.
On the top it does say (General) on the left and Workbook_Open on the right.
Will someone please tell me what I'm doing wrong?

Thanks so much.

Sub Workbook_Open()
     Workbooks.Open Filename:="A:\B:.xlsm"
End Sub