run addin macro on open workbook - excel

Question

I have seen where you can run a macro based on inserted text, but the macro has to be embedded in the sheet, i.e., Private Sub Worksheet_Change(ByVal Target As Range).
What I want to do is call the macro from my add-in from a user's workbook that will not have "Worksheet_Change..." already embedded at the sheet level. Is there a way to do this?
As for additional background, I know I can run the macro from the add in, but I want to activate it using a bar-code scan rather than calling the macro from a button or some other interface.
My bar-code reads as Make Landscape 1 pg when scanned. Hoping to use some modification of this:
Private Sub Worksheet_Change(ByVal Target As Range)
If (Target.Value) = "Make Landscape 1 pg" Then
With Application
.EnableEvents = False
.Undo
.EnableEvents = True
End With
Application.ScreenUpdating = False
'ActiveWindow.SelectedSheets.PrintPreview
ActiveSheet.Select
With ActiveSheet.PageSetup
.Orientation = xlLandscape
.FitToPagesWide = 1
.FitToPagesTall = 1
End With
Application.ScreenUpdating = True
End If
End Sub

Answer 1

Here's a link that I found very useful when needing to access Worksheet/Workbook-level events via modules in my Add-In. The basic implementation is like so:
In Add-In's ThisWorkbook module:
Private WithEvents App As Application
Private Sub Workbook_Open()
Set App = Application
End Sub
Now App can be used to call/access workbook & worksheet events. If you're trying to check something upon selection change from a module in an add-in it has to be Workbook_Sheetselectionchange event (which you can read more about here) instead of the Worksheet_Change event. This event can be used in conjunction with the previously set App variable like so:
In Add-In's ThisWorkbook module:
Private Sub App_SheetSelectionChange(ByVal Sh As Object, _
ByVal Target As Excel.Range)
If (Target.Value) = "Make Landscape 1 pg" Then
With Application
.EnableEvents = False
.Undo
.EnableEvents = True
End With
Application.ScreenUpdating = False
ActiveSheet.Select
With ActiveSheet.PageSetup
.Orientation = xlLandscape
.FitToPagesWide = 1
.FitToPagesTall = 1
End With
Application.ScreenUpdating = True
End If
End Sub