public static string RequestServer(string methodName, List<string> parameters)
{
    string ServerIp = "http://localhost:18332";
    string UserName = "user name goes here";
    string Password = "password goes here";

    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(ServerIp);
    webRequest.Credentials = new NetworkCredential(UserName, Password);

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

    string respVal = string.Empty;

    JObject joe = new JObject();
    joe.Add(new JProperty("jsonrpc", "1.0"));
    joe.Add(new JProperty("id", "1"));
    joe.Add(new JProperty("method", methodName));

    JArray props = new JArray();
    foreach (var parameter in parameters)
    {
        props.Add(parameter);
    }

    joe.Add(new JProperty("params", props));

    // serialize json for the request
    string s = JsonConvert.SerializeObject(joe);
    byte[] byteArray = Encoding.UTF8.GetBytes(s);
    webRequest.ContentLength = byteArray.Length;

    Stream dataStream = webRequest.GetRequestStream();
    dataStream.Write(byteArray, 0, byteArray.Length);
    dataStream.Close();

    StreamReader streamReader = null;
    try
    {
        WebResponse webResponse = webRequest.GetResponse();

        streamReader = new StreamReader(webResponse.GetResponseStream(), true);

        respVal = streamReader.ReadToEnd();
        var data = JsonConvert.DeserializeObject(respVal).ToString();
        return data;
    }
    catch (Exception exp)
    {
        throw (exp);
    }
    finally
    {
        if (streamReader != null)
        {
            streamReader.Close();
        }
    }

    return string.Empty;
}

double AmountToSend = Convert.ToDouble(txtAmountToSend.Text);
string ReceivingAddress = txtSendToAddress.Text;
JObject data = JObject.Parse(RequestServer("sendtoaddress", new List<string>() { ReceivingAddress, Convert.ToString(AmountToSend) }));

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