detect Alt Tab in my app

Try this: Make a Form with a Timer and set it's Interval to 1.

Private Declare Function GetAsyncKeyState Lib "user32" (ByVal vKey As Long) As Integer
Private Const VK_MENU = &H12
Private Const VK_TAB = &H9

Private Sub Timer1_Timer()

    If GetAsyncKeyState(VK_MENU) And GetAsyncKeyState(VK_TAB) Then
        Print "Alt Tab as pressed"
    End If
    
End Sub