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type
  TForm1 = class(TForm)
    procedure Hallo(var Msg: tagMsg); message WM_APPCOMMAND;
  end;

{...}

procedure TForm1.Hallo(var Msg: tagMSG);
begin
  case Msg.wParam of
    720896: ShowMessage('Nächste Folie');
    786432: ShowMessage('Vorherige Folie');
    917504: ShowMessage('Leere Folie');
  end;
end;					

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  {$EXTERNALSYM WM_APPCOMMAND}
  WM_APPCOMMAND       = $0319;					

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WM_APPCOMMAND       = $0319;