How to use regular C++ code in GUI written in CLR?

Question

visual studio

Here I created the project "CLR Application", created the form and stuck the button.
Here is the button event:

private: System::Void button1_Click(System::Object^  sender, System::EventArgs^  e) {
    ...
  }

I want to display a MessageBox , not the one that the CLR has, but the one in Windows.h , how can I do this?