Thread^ ChatThread = gcnew Thread(gcnew ThreadStart(this, &MagicBox2017::MainMenu::Chat));
    ChatThread->SetApartmentState(ApartmentState::STA);
    ChatThread->Start();

void MagicBox2017::MainMenu::Chat(){
    WebBrowser^ br = gcnew WebBrowser();
    this->Invoke(gcnew MethodInvoker(this, &MainMenu::newBrowserParent));
    br->Location = Point(1, 1);
    br->Dock = System::Windows::Forms::DockStyle::Fill;
    br->Size = System::Drawing::Size(255, 259);
    br->ScrollBarsEnabled = false;
    br->WebBrowserShortcutsEnabled = false;
    br->BringToFront();
    this->Controls->Add(br);
    //br->Navigate("index.html");
    chatBrowser->Navigate("http://forum.superpower2.ru/MagicBox/chat/ajax-chat.html");
}

void newBrowserParent(){
      br->Parent = p_chat; //но тут не существует br
    }