how to send data via com port while pressing a button?

In order to access comport you need to have namespace
using System.IO.Ports;
then in button click event write this
{
then need to create an object for serial port like this

SerialPort obj = new SerialPort("COM1");//But be shore that the bardrate matches between hardware and computer system 
            obj.Open();

 obj.Write("some data");
            obj.Close();

}

Hope this may help you