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You can retrieve the window handle that can be used for the function by casting the Outlook window instance like Explorer or Inspector to the IOLEWindow interface and using the IOleWindow::GetWindow method which retrieves a handle to one of the windows participating in in-place activation (frame, document, parent, or in-place object window).

Here is the sample code which shows how to use the IOLEWindow interface with Outlook windows:

/// <summary>
 /// Implemented and used by containers and objects to obtain window handles 
 /// and manage context-sensitive help.
 /// </summary>
 /// <remarks>
 /// The IOleWindow interface provides methods that allow an application to obtain  
 /// the handle to the various windows that participate in in-place activation, 
 /// and also to enter and exit context-sensitive help mode.
 /// </remarks>
 [ComImport]
 [Guid("00000114-0000-0000-C000-000000000046")]
 [InterfaceType (ComInterfaceType.InterfaceIsIUnknown)]
 public interface IOleWindow
 {
     /// <summary>
     /// Returns the window handle to one of the windows participating in in-place activation 
     /// (frame, document, parent, or in-place object window).
     /// </summary>
     /// <param name="phwnd">Pointer to where to return the window handle.</param>
     void GetWindow (out IntPtr phwnd) ;

     /// <summary>
     /// Determines whether context-sensitive help mode should be entered during an 
     /// in-place activation session.
     /// </summary>
     /// <param name="fEnterMode"><c>true</c> if help mode should be entered; 
     /// <c>false</c> if it should be exited.</param>
     void ContextSensitiveHelp ([In, MarshalAs(UnmanagedType.Bool)] bool fEnterMode) ;
 }

 public void getWindowHandle()
 {
        dynamic activeWindow = Globals.ThisAddIn.Application.ActiveWindow();
        IOleWindow win = activeWindow as IOleWindow;
        IntPtr window = IntPtr.Zero; 
        win.GetWindow(window);
 }