从actionresult重定向到支付快递网站

我正试图在使用时打开付款快递网站，点击账单提交按钮。为此，我写了这个代码

 [HttpPost]
        public ActionResult Billing()   
        {
            string URI = ConfigurationManager.AppSettings["paymentexpressUrl"].ToString();
            var PxPayUserId = ConfigurationManager.AppSettings["PxPayUserId"].ToString();
            var PxPayKey = ConfigurationManager.AppSettings["PxPayKey"].ToString();
            // form the PXPost Xml message
            StringWriter sw = new StringWriter();
            XmlTextWriter xtw = new XmlTextWriter(sw);
            xtw.WriteStartElement("Txn");
            xtw.WriteElementString("PostUsername", PxPayUserId);
            xtw.WriteElementString("PostPassword", PxPayKey);
            xtw.WriteElementString("Amount", "100");
            xtw.WriteElementString("InputCurrency", "USD");
            xtw.WriteElementString("TxnType", "Purchase");
            xtw.WriteElementString("TxnId", "");
            xtw.WriteElementString("MerchantReference", "Test Transaction");
            xtw.WriteEndElement();
            xtw.Close();

            // Send the Xml message to PXPost
            WebRequest wrq = WebRequest.Create(URI);
            wrq.Method = "POST";
            wrq.ContentType = "application/x-www-form-urlencoded";
            byte[] b = Encoding.ASCII.GetBytes(sw.ToString());
            wrq.ContentLength = b.Length;
            Stream s = wrq.GetRequestStream();
            s.Write(b, 0, b.Length);
            s.Close();
            return wrq;
        }

但不知道如何重定向到支付快递网站。我怎么能做到这一点。