Can't create workitem via webrequest in RTC using C#

CookieContainer cookies = new CookieContainer();
HttpWebRequest documentPost = (HttpWebRequest)WebRequest.Create(rtcServerUrl + "/oslc/contexts/_0_iN4G09EeGGMqpyZT5XdQ/workitems/order");//"Order" is the workitem name
documentPost.Method = "POST";
documentPost.CookieContainer = cookies;
documentPost.Accept = "application/json";
documentPost.ContentType = "application/x-oslc-cm-change-request+json";
documentPost.Timeout = TIMEOUT_SERVICO;
string json = "{ \"dc:title\":\"" + title + "\", \"rtc_cm:filedAgainst\": [     { \"rdf:resource\" : \"" + rtcServerUrl + "/resource/itemOid/com.ibm.team.workitem.Category/" + idCategory + "\"} ] }"; //dc:title and rtc_cm:filedAgainst are the only two mandatory data from the workitem I'm trying to create
using (var writer = new StreamWriter(documentPost.GetRequestStream()))
{
    writer.Write(json);
    writer.Flush();
    writer.Close();
}
Encoding encode = System.Text.Encoding.UTF8;
string retorno = null;
//Login
HttpWebRequest formPost = (HttpWebRequest)WebRequest.Create(rtcServerUrl +     "/j_security_check");
formPost.Method = "POST";
formPost.Timeout = TIMEOUT_REQUEST;
formPost.CookieContainer = request.CookieContainer;
formPost.Accept = "text/xml";
formPost.ContentType = "application/x-www-form-urlencoded";
String authString = "j_username=" + userName + "&j_password=" + password;     //create authentication string
Byte[] outBuffer = System.Text.Encoding.UTF8.GetBytes(authString); //store in byte buffer
formPost.ContentLength = outBuffer.Length;
System.IO.Stream str = formPost.GetRequestStream();
str.Write(outBuffer, 0, outBuffer.Length); //update form
str.Close();
//FormBasedAuth Step2:submit the login form and get the response from the     server
HttpWebResponse formResponse = (HttpWebResponse)formPost.GetResponse();
var rtcAuthHeader = formResponse.Headers["X-com-ibm-team-repository-web-    auth-msg"];
//check if authentication has failed
if ((rtcAuthHeader != null) && rtcAuthHeader.Equals("authfailed"))
{
    //authentication failed. You can write code to handle the authentication     failure.
    //if (DEBUG) Console.WriteLine("Authentication Failure");
}
else
{
    //login successful
    HttpWebResponse responseRetorno =     (HttpWebResponse)documentPost.GetResponse();
    if (responseRetorno.StatusCode == HttpStatusCode.Created)
    {
        retorno = "[Header] " + responseRetorno.GetResponseHeader("Location") + System.Environment.NewLine;
    }else
    {
        StreamReader reader = new     StreamReader(responseRetorno.GetResponseStream());
        retorno = "[Response] "  + reader.ReadToEnd();
     }
    responseRetorno.Close();
    formResponse.GetResponseStream().Flush();
    formResponse.Close();
}