[vb2008] Press the button of an external not foreground / minimized window

[phil2000]

known the external window size, it Hwnd and the x/y coordinates of the button to press, I am trying this code, like seen on the web:

    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function PostMessage(ByVal hWnd As IntPtr, _
    ByVal Msg As  UInteger, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As Boolean
    End Function

    Const WM_LBUTTONDOWN As UInteger = &H201

    'button X coordinates, tried both X => respect to it window coordinates
    'and X respect to Screen coordinates (when the window is not minimized)
    Dim X as Integer = ....     

    'button Y coordinates, tried both Y => respect to it window coordinates 
    'and Y respect to Screen coordinates (when the window is not minimized)
    Dim Y as Integer = ....   
 
    Dim Hwnd as IntPtr = ...   ' external window handle

    PostMessage(Hwnd, WM_LBUTTONDOWN, IntPtr.Zero, MakeLParam(X, Y))


    Private Function MakeLParam(ByVal LoWord As Integer, ByVal HiWord As Integer) As IntPtr

        Return New IntPtr((HiWord << 16) Or (LoWord And &HFFFF))

    End Function

but nothing happens, both if the external window is in foreground, NOT in foreground or minimized...

what's wrong?

ty