Code:
function FindWindowByTitle(WindowTitle: string): Hwnd;
var
  NextHandle: Hwnd;
  NextTitle: array[0..260] of char;
begin

  NextHandle := GetWindow(Application.Handle, GW_HWNDFIRST);
  while NextHandle > 0 do
  begin

    GetWindowText(NextHandle, NextTitle, 255);
    if Pos(WindowTitle, StrPas(NextTitle)) <> 0 then
    begin
      Result := NextHandle;
      Exit;
    end
    else

      NextHandle := GetWindow(NextHandle, GW_HWNDNEXT);
  end;
  Result := 0;
end;

Code:
procedure TForm1.Button1Click(Sender: TObject);
  function EnumChildProc(AHandle: THandle; AItems: TStrings): BOOL; stdcall;
  var
    buffer: array[0..255] of Char;
    caption: array[0..255] of Char;
  begin
    Result := True;
    GetClassName(AHandle, buffer, SizeOf(buffer) - 1);
    SendMessage(AHandle, WM_GETTEXT, 256, Integer(@caption));

    AItems.Add(Format('Handle: %d, Class: %s, Text: %s', [AHandle, buffer, caption]));
  end;

var
  wHnd: THandle;
begin
  wHnd := FindWindowByTitle('Program');
  ListBox1.Clear;

  EnumChildWindows(wHnd, @EnumChildProc, Integer(ListBox1.Items));

end;

Code:
  SendMessage(handle, WM_SETTEXT, 0, Integer(PChar('tekst')));