Run Macro when file window is opened

Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long

Declare Function CallWindowProc Lib "user32" Alias "CallWindowProcA" _
(ByVal lpPrevWndFunc As Long, ByVal hwnd As Long, _
ByVal Msg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long

Const GWL_WNDPROC  As Long = (-4)
Const WM_ACTIVATEAPP   As Long = &H1C

Dim OldWindowProc As Long
Dim hwnd As Long
Dim blnXLIsSubclassed As Boolean


Sub SubClass()

    'Subclass The Excel Application Window
    If Not blnXLIsSubclassed Then
        hwnd = Application.hwnd
        OldWindowProc = SetWindowLong(hwnd, GWL_WNDPROC, AddressOf NewWindowProc)
        blnXLIsSubclassed = True
    End If
End Sub


Sub UnSubclass()

    'UnSubclass The Excel Application Window
    SetWindowLong hwnd, GWL_WNDPROC, OldWindowProc
    blnXLIsSubclassed = False
    
End Sub


Function NewWindowProc(ByVal hwnd As Long, ByVal Msg _
    As Long, ByVal wParam As Long, ByVal lParam As Long) As _
    Long
    
    Dim blnXLActivated As Boolean
    If Msg = WM_ACTIVATEAPP Then
        blnXLActivated = wParam
        If blnXLActivated = True Then
            ActiveSheet.Paste Range("b4") '\\ Do other stuff too here
        End If
        Exit Function
    End If
    NewWindowProc = CallWindowProc(OldWindowProc, hwnd, Msg, wParam, lParam)
   
End Function