Reading External File On Workbook Open

when i run this code under a button on a sheet it works fine. but i would like to run when the workbook opens (but i get an error message

On Error Resume Next
Application. ScreenUpdating = False
Dim wb As Workbook
Dim ws As Worksheet
'create new work order number...
Set wb = Application.Workbooks.Open("c: empex_ExternalOrderNumber.xls")
Set ws = Worksheets("NumberIncrement")
'get new WO number from numberincrement.xls
If Err.Number <> 0 Then
MsgBox Err.Description & "...help"
Else
mynewnumber = ws. Range("b1").Value
Range("g5").Value = mynewnumber
wb.Close
End If

Application.ScreenUpdating = True