try

            {

                StringBuilder data = new StringBuilder();

                data.Append("oid=[youroid]");

                data.Append("&first_name=" + Server.UrlEncode(this.txtFirstName.Text));

                data.Append("&last_name=" + Server.UrlEncode(this.txtLastName.Text));

                data.Append("&email=" + Server.UrlEncode(this.txtEmail.Text));

                string url = "https://test.salesforce.com/servlet/servlet.WebToLead?encoding=UTF-8";

                HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);

                req.Method = "POST";

                req.ContentType = "application/x-www-form-urlencoded";

                byte[] buffer = Encoding.UTF8.GetBytes(data.ToString());

                req.ContentLength = buffer.Length;

                using (Stream reqst = req.GetRequestStream())

                {

                    reqst.Write(buffer, , buffer.Length);

                }

                WebResponse response = req.GetResponse();

            }

            catch (Exception ex)

            {

            }