procedure TForm1.Button1Click(Sender: TObject);
var
  CDS: TCopyDataStruct;
  str:string;
begin
  str := 'message1';

  CDS.dwData := 1;

  CDS.cbData := Length(str) + 1;

  GetMem(CDS.lpData, CDS.cbData);

  StrPCopy(CDS.lpData, AnsiString(str));

  SendMessage(FindWindow(nil, 'Receiver'),
                  WM_COPYDATA, Handle, Integer(@CDS));

  FreeMem(CDS.lpData, CDS.cbData);

 end;

procedure TForm1.WMCopyData(var MessageData: TWMCopyData);
var
  s:string;
begin
  if MessageData.CopyDataStruct.dwData = 1 then
  begin
    s := PAnsiChar((MessageData.CopyDataStruct.lpData));
    Label1.Caption:=s;
 //   ShowMessage(S);
    MessageData.Result := 123;
  end
  else
    MessageData.Result := -321;
end;

FindWindow(nil, 'Receiver')

    SendMessage(receiverHandle, WM_LBUTTONDOWN, 300,300);
    SendMessage(receiverHandle, WM_LBUTTONup, 300,300);
    SetWindowPos(receiverHandle, HWND_BOTTOM, 1, 1, 20, 20, swp_nosize);

procedure WMCopyData(var MessageData: TWMCopyData); message WM_COPYDATA; //это есть?

var si: TStartupInfo;
    pi: TProcessInformation;
    ExitCode: DWORD;
begin
  Result := True;
  // запускаем проверку архива
  si := Default(TStartupInfo);
  if not CreateProcess(nil, PChar(string(AppPath)),
                       nil, nil, False, CREATE_NO_WINDOW, nil, nil, si, pi) then
    raise Err('Не удалось запустить');
  // и ждем, пока процесс отработает
  try
    repeat
      if WaitForSingleObject(pi.hProcess, 5*MSecsPerSec) = WAIT_TIMEOUT then
        begin TerminateProcess(pi.hProcess, 0); raise Err('Превышен срок ожидания'); end;
      if not GetExitCodeProcess(pi.hProcess, ExitCode) then Break;
    until ExitCode <> STILL_ACTIVE;
  finally
    CloseHandle(pi.hProcess); CloseHandle(pi.hThread);
  end;
  // проверяем код завершения проги. 0 - ОК, иначе - ошибка
  Result := (ExitCode = 0);