Yardım xml 'e textboxdan veri ekleme

Elimde sms firmasından aldığım api var programa ekliyorum sms atıyor. ama benim istediğim mesaj metni ve cep telefon numarasını textboxdan çekmek bir türlü yapamadım yardımcı olabilirmisiniz

Mesaj metni
0

905303626490
905051234567

public static void CallWebService()
{
var _url = “https://webservice.asistiletisim.com.tr/SmsProxy.asmx”;
var _action = “https://webservice.asistiletisim.com.tr/SmsProxy/sendSms”;

        XmlDocument soapEnvelopeXml = new XmlDocument();
        soapEnvelopeXml.LoadXml(@"<soapenv:Envelope xmlns:soapenv=""http://schemas.xmlsoap.org/soap/envelope/"" xmlns=""https://webservice.asistiletisim.com.tr/SmsProxy"">
                                       <soapenv:Header/>
                                       <soapenv:Body>
                                          <sendSms>
                                             <requestXml><![CDATA[<SendSms>
                                                   <Username>xxxx</Username>
                                                   <Password>xxxxx</Password>
                                                   <UserCode>xxxxx</UserCode>
                                                   <AccountId>xxxxxx</AccountId>
                                                   <Originator>xxxxxx</Originator>
                                                   <ValidityPeriod>60</ValidityPeriod>
                                                   <SendDate/>
                                                   <MessageText>This is one to many sending example..</MessageText>
                                                   <IsCheckBlackList>0</IsCheckBlackList> 
                                                      <ReceiverList>
                                                      <Receiver>905303626490</Receiver>
                                                      <Receiver>905051234567</Receiver>
                                                   </ReceiverList>
                                                </SendSms>]]></requestXml>
                                          </sendSms>
                                       </soapenv:Body>
                                    </soapenv:Envelope>");

        HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(_url);
        webRequest.Headers.Add("SOAPAction", _action);
        webRequest.ContentType = "text/xml;charset=\"utf-8\"";
        webRequest.Accept = "text/xml";
        webRequest.Method = "POST";

        using (Stream stream = webRequest.GetRequestStream())
        {
            soapEnvelopeXml.Save(stream);
        }

        IAsyncResult asyncResult = webRequest.BeginGetResponse(null, null);
        asyncResult.AsyncWaitHandle.WaitOne();

        string soapResult;
        using (WebResponse webResponse = webRequest.EndGetResponse(asyncResult))
        {
            using (StreamReader rd = new StreamReader(webResponse.GetResponseStream()))
            {
                soapResult = rd.ReadToEnd();
            }

            Console.Write(soapResult);
        }
    }