Visual studio gui with serial communication, how other cpp would communicate?

See more:

C++

I have made a GUI in Windows with Visual Studio, and that GUI has a working serial communication. Now I added an another cpp file to project and I would like that cpp to communicate with already opened com port. What would be the easiest way for absolute beginner to try to make that work? Or is it even possible?

What I have tried:

MyForm.h (gui) (shortened):

using namespace System;
    using namespace System::IO::Ports;

public ref class MyForm : public System::Windows::Forms::Form
    {
    public:
        MyForm(void)
        {
            InitializeComponent();
            findPorts();
        }

protected:
private: System::IO::Ports::SerialPort^ serialPort1;

#pragma region Windows Form Designer generated code  
        void InitializeComponent(void)
        {
          this->components = (gcnew System::ComponentModel::Container());
          this->serialPort1 = (gcnew System::IO::Ports::SerialPort(this->components));
        }
#pragma endregion
    private: void findPorts(void)
    {
        array<Object^>^ objectArray = SerialPort::GetPortNames();
    }
private: System::Void button7_Click(System::Object^ sender, System::EventArgs^ e) {
    //add sender name
    String^ name = this->serialPort1->PortName;
    //grab text and store in send buffer
    String^ message = "hello";
    //write to serial
    if (this->serialPort1->IsOpen)
        this->serialPort1->WriteLine(message);
    else
        this->textBox2->Text = "Port Not Opened";
}