procedure TForm1.Button1Click(Sender: TObject);
var buf: array [0..255] of char;
begin
  Sendmessage(combobox1.Handle,WM_LBUTTONDOWN, MK_LBUTTON, 0);
  Sendmessage(combobox1.Handle,WM_LBUTTONUP, 0, 0);
  Sendmessage(combobox1.Handle,WM_KEYDOWN, VK_DOWN, 0);
  Sendmessage(combobox1.Handle,WM_KEYUP, VK_DOWN, 0);
  getwindowtext(combobox1.Handle, @buf, sizeof(buf));
  showmessage(string(buf));

end;