string parameters = String.Format("apicid={0}&apikey={1}&product={2}&firstname={3}&lastname={4}&street={5}&hno
                    ={6}&zip={7}&city={8}&country={9}&phone={10}",
                 apicid, apikey, product,conna2, conna1,street, hno, zip, city, country, phone);

       string response = AdressLaborWebRequest(parameters);

        private string AdressLaborWebRequest(string parameter)
        {
            try
            {
                HttpWebRequest req = WebRequest.Create(new Uri("https://api.adresslabor.de/v1/de/check")) as HttpWebRequest;
                req.KeepAlive = false;
                // pass params to the service
                req.Method = "POST";
                req.ContentType = "application/x-www-form-urlencoded";
                byte[] para = Encoding.UTF8.GetBytes(parameter);
                req.ContentLength = para.Length;
                Stream strm = req.GetRequestStream();
                strm.Write(para, 0, para.Length);
                strm.Close();
                // read response from the service
                HttpWebResponse resp = req.GetResponse() as HttpWebResponse;
                StreamReader result = new StreamReader(resp.GetResponseStream(), Encoding.UTF8); 
                string response = result.ReadToEnd();
                result.Close();
                resp.Close();
                return response;
            }
            catch (WebException ex)
            {
                return "";
            }
        }