How to do google captcha in form submit in mvc controller

 string EncodedResponse = Request.Form["g-Recaptcha-Response"];
ReCaptchaClass.Validate(EncodedResponse, Request)



 public class ReCaptchaClass
    {
        public static string Validate(string EncodedResponse, HttpRequestBase request)
        {
            var client = new WebClient();
           
            if (request.Url.Host == "www-mot-gov-sg.cwp.sg" || request.Url.Host == "www.mot.gov.sg")
            {
                client.Proxy.Credentials = new NetworkCredential("www.mot.gov.sg", "NdjdrkhtvTze");
            }
            else if (request.Url.Host == "www-mot-gov-sg.cwp-stg.sg")
            {
                client.Proxy.Credentials = new NetworkCredential("www.mot.gov.sg", "YcJyMvMqxgr3");
            }
       
            string PrivateKey = "6LdmLEEUAAAAALihIQfuD--P0wVUD3Mieb2lex81";

            if(request.Url.Host == "localhost")
            {
                PrivateKey = "6Lf5BD4UAAAAAOqvZqtBlhHtqZSx7GZcfqOOJj-P";
            }

            var GoogleReply = client.DownloadString(string.Format("https://www.google.com/recaptcha/api/siteverify?secret={0}&response={1}", PrivateKey, EncodedResponse));

            var captchaResponse = Newtonsoft.Json.JsonConvert.DeserializeObject<ReCaptchaClass>(GoogleReply);

            return captchaResponse.Success;
        }

        [JsonProperty("success")]
        public string Success
        {
            get { return m_Success; }
            set { m_Success = value; }
        }

        private string m_Success;
        [JsonProperty("error-codes")]
        public List<string> ErrorCodes
        {
            get { return m_ErrorCodes; }
            set { m_ErrorCodes = value; }
        }


        private List<string> m_ErrorCodes;
    }