Client:
private void SendButton_Click(object sender, EventArgs e)
{
			decimal FirstNumber = FirstNumberNumericUpDown.Value;
            decimal SecondNumber = SecondNumberNumericUpDown.Value;
            string Operator = string.Empty;

            if (AdditonRadioButton.Checked)
            {
                Operator = "+";
            }
            else if (SubtractionRadioButton.Checked)
            {
                Operator = "-";
            }
            else if (MultiplicationRadioButton.Checked)
            {
                Operator = "*";
            }
            else if (DivisionRadioButton.Checked)
            {
                Operator = "/";
            }

            string result = string.Empty;
            HttpWebRequest webRequest;

            string requestParams = "{\"FirstNumber\": "+ FirstNumber.ToString() +", \"SecondNumber\": " + SecondNumber.ToString() + ",\"Operator\": \"" + Operator + "\"}";

            webRequest = (HttpWebRequest)WebRequest.Create(UrlTextBox1.Text);

            webRequest.Method = "POST";
            webRequest.ContentType = "application/json";

            byte[] byteArray = Encoding.UTF8.GetBytes(requestParams);
            webRequest.ContentLength = byteArray.Length;
            try
            {
                using (Stream requestStream = webRequest.GetRequestStream())
                {
                    requestStream.Write(byteArray, 0, byteArray.Length);
                }
            }
            catch(Exception ex)
            {
                listBox.Items.Add("Could not connect to the server");
                listBox.Items.Add("Exception: " + ex);
                return;
            }
}

Server:

        public void Post(Equation equation)
        {

            string directory = HttpRuntime.AppDomainAppPath;

            string text = equation.FirstNumber.ToString() + " " + equation.SecondNumber.ToString() + " " + equation.Operator;
            System.IO.File.WriteAllText(directory + "txtfile.txt", text);
        }