YahooOAuthBase oauth = new YahooOAuthBase();

            Uri uri = new Uri("http://social.yahooapis.com/v1/user/" + OauthYahooGuid + "/contacts?format=XML");
            string nonce = oauth.GenerateNonce();
            string timeStamp = oauth.GenerateTimeStamp();
            string normalizedUrl;
            string normalizedRequestParameters;
            string sig = oauth.GenerateSignature(uri, ConsumerKey, ConsumerSecret, OauthToken, OauthTokenSecret, "POST", timeStamp, nonce,  YahooOAuthBase.SignatureTypes.HMACSHA1, out normalizedUrl, out normalizedRequestParameters);


            try
            {
                StreamReader sr = new StreamReader(Server.MapPath("~/YahooAddRequest.xml"));
                string requestData=sr.ReadToEnd();
                
                HttpWebRequest req = (HttpWebRequest)WebRequest.Create(uri.ToString());
                req.Method = "POST";
                
                req.ContentType = "application/xml";


                string authHeader = "Authorization: OAuth " +
                "realm=\"yahooapis.com\"" +
                ",oauth_consumer_key=\"" + ConsumerKey + "\"" +
                ",oauth_nonce=\"" + nonce + "\"" +
                ",oauth_signature_method=\"HMAC-SHA1\"" +
                ",oauth_timestamp=\"" + timeStamp + "\"" +
                ",oauth_token=\"" + OauthToken + "\"" +
                ",oauth_version=\"1.0\"" +
                ",oauth_signature=\"" + HttpUtility.UrlEncode(sig) + "\"";


                req.Headers.Add(authHeader);


                req.ContentLength =requestData.Length;
                Stream reqStream = req.GetRequestStream();


                StreamWriter sw = new StreamWriter(reqStream);

                sw.Write(requestData);
                sw.Close();

                

                HttpWebResponse res = (HttpWebResponse)req.GetResponse();

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<contact>
  <fields>
    <type>nickname</type>
    <value>Banana2</value>
  </fields>
  <fields>
    <type>notes</type>
    <value>Loooser!</value>
  </fields>
  <fields>
    <type>name</type>
    <value>
      <givenName>Test</givenName>
      <familyName>Contact</familyName>
    </value>
  </fields>
  <fields>
    <type>email</type>
    <value>demo@demo1.com</value>
    <flags>WORK</flags>
  </fields>
  <fields>
    <type>email</type>
    <value>something@some.com</value>
    <flags>WORK</flags>
  </fields>
  <fields>
    <type>phone</type>
    <value>234342424</value>
    <flags>WORK</flags>
  </fields>
  <fields>
    <type>phone</type>
    <value>1211 12333</value>
    <flags>WORK</flags>
  </fields>
  <fields>
    <type>jobtitle</type>
    <value>lead idiot</value>
  </fields>
  <fields>
    <type>company</type>
    <value>bannerama</value>
  </fields>
  <categories>
    <name>friend</name>
  </categories>
</contact>