"Worksheet_Change" from Add-In

I'm trying to run a macro for a selection change. The unique aspect ... it's in an Add-In.
I've placed the following in Sheet 1, saved the Add-In, turned on the Add-In.
It does not run.

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim KeyCells As Range
    Set KeyCells = Range("A1:Z100")
    
    If Not Application.Intersect(KeyCells, Range(Target.Address)) _
           Is Nothing Then
        MsgBox "Cell " & Target.Address & " has changed."
    End If
End Sub