Test.EXE /p 4711

function WndProc(hWnd: HWND; uMsg: UINT; wParam: wParam; lParam: LParam):
  lResult; stdcall;
begin
  Result := 0;

  case uMsg of
    WM_CREATE:
      begin
        if ScreenMode = scrmStart then ShowCursor(FALSE);
        SetTimer(hWnd, 1, 100, nil);
      end;
    WM_DESTROY:
      begin
        if ScreenMode = scrmStart then ShowCursor(TRUE);
        KillTimer(hWnd, 1);
        DestroyWindow(hWnd);
        PostQuitMessage(0);
      end;
    WM_ACTIVATE:
      if (wParam = Integer(FALSE)) then
        PostMessage(hWnd, WM_CLOSE, 0, 0); // terminates the screen saver.

    WM_LBUTTONDOWN,
    WM_MBUTTONDOWN,
    WM_RBUTTONDOWN,
    WM_KEYDOWN:
      begin
        if ScreenMode = scrmStart then
          PostMessage(hWnd, WM_CLOSE, 0, 0);
      end;

    // Wird verwendet um den Start des Bildschirmschoners zu verhindern
    // zB. wenn eine App. den Bildschirmsch. unterdrücken will
    SC_SCREENSAVE,
    SC_CLOSE: Result := 0;

    WM_TIMER:
      begin
        OnTimerEvent(hWnd);
      end;
  else
    Result := DefWindowProc(hWnd, uMsg, wParam, lParam);
  end;
end;

BEGIN
  InitCommonControls; // Wegen XPManifest in der Res.

  // Comandline auswerten
  if (ParamStr(1) + #32)[1] in ['-', '/'] then
  begin
    case (ParamStr(1))[2] of
      'S', 's': ScreenMode := scrmStart;
      'P', 'p': ScreenMode := scrmPreview;
      'C', 'c': ScreenMode := scrmConfig;
    end;
  end;
  if ParamCount = 0 then ScreenMode := scrmStart;

  // Create Window
  if System.HPrevInst = 0 then
  begin
    ZeroMemory(@WndClass, Sizeof(TWndClass));

    with WndClass do
    begin
      Style := CS_VREDRAW or CS_HREDRAW or CS_SAVEBITS or CS_DBLCLKS;
      lpfnWndProc := @WndProc;
      cbClsExtra := 0;
      cbWndExtra := 0;
      hbrBackground := GetStockObject(BLACK_BRUSH);
      lpszMenuName := nil;
      lpszClassName := WndClassName;
      hInstance := hInstance;
      hIcon := LoadIcon(0, IDI_APPLICATION);
      hCursor := LoadCursor(0, IDC_ARROW);
    end;
    RegisterClass(WndClass);
  end;

  if ScreenMode = scrmStart then
  begin
    GetClientRect(GetDesktopWindow, WndRect);
    AppHWND :=
      CreateWindow(
        WndClassName,
        AppName,
        WS_VISIBLE or WS_POPUP or WS_EX_TOPMOST {or WS_SYSMENU},
        WndRect.Left,
        WndRect.Top,
        WndRect.Right,
        WndRect.Bottom,
        0,
        0,
        hInstance,
        nil);
  end else
  if ScreenMode = scrmPreview then
  begin
    ParentWnd := _StrToInt(ParamStr(2)); // Get Window Handle // <---<<<
    GetClientRect(ParentWnd, WndRect);
    AppHWND :=
      CreateWindow(
        WndClassName,
        AppName,
        WS_VISIBLE or WS_POPUP,
        WndRect.Left,
        WndRect.Top,
        WndRect.Right,
        WndRect.Bottom,
        0, {ParentWnd,} // <---<<<
        0,
        hInstance,
        nil);
    SetParent(AppHWND, ParentWnd); // <---<<<
  end else // elseif ScreenMode = scrmConfig
  begin
    DialogBoxParam(hInstance, MAKEINTRESOURCE(IDD_DLG), AppHWND, @DlgProc,0);
    PostQuitMessage(0);
  end;

  UpdateWindow(AppHWND);

  // "Application.Run"
  while GetMessage(msg, 0, 0, 0) do
  begin
    TranslateMessage(msg);
    DispatchMessage(msg);
  end;

END.