just continuing what @Mulki made with his code

public string WebRequestinJson(string url, string postData)
{
    string ret = string.Empty;

    StreamWriter requestWriter;

    var webRequest = System.Net.WebRequest.Create(url) as HttpWebRequest;
    if (webRequest != null)
    {
        webRequest.Method = "POST";
        webRequest.ServicePoint.Expect100Continue = false;
        webRequest.Timeout = 20000;

        webRequest.ContentType = "application/json";
        //POST the data.
        using (requestWriter = new StreamWriter(webRequest.GetRequestStream()))
        {
            requestWriter.Write(postData);
        }
    }

    resp = (HttpWebResponse)webRequest.GetResponse();
    Stream resStream = resp.GetResponseStream();
    StreamReader reader = new StreamReader(resStream);
    ret = reader.ReadToEnd();

    return ret;
}

just continuing what @Mulki made with his code

public string WebRequestinJson(string url, string postData)
{
    string ret = string.Empty;

    StreamWriter requestWriter;

    var webRequest = System.Net.WebRequest.Create(url) as HttpWebRequest;
    if (webRequest != null)
    {
        webRequest.Method = "POST";
        webRequest.ServicePoint.Expect100Continue = false;
        webRequest.Timeout = 20000;

        webRequest.ContentType = "application/json";
        //POST the data.
        using (requestWriter = new StreamWriter(webRequest.GetRequestStream()))
        {
            requestWriter.Write(postData);
        }
    }

    HttpWebResponse resp = (HttpWebResponse)webRequest.GetResponse();
    Stream resStream = resp.GetResponseStream();
    StreamReader reader = new StreamReader(resStream);
    ret = reader.ReadToEnd();

    return ret;
}