var request = (HttpWebRequest)WebRequest.Create("http://sitio/webservices/auth/token/");

var postData = "grant_type=grant_type&username=username&password=password&id=id&secret=secret";

//    request.Headers.Add("id", "id");
//    request.Headers.Add("secret", "secret");

var data = Encoding.ASCII.GetBytes(postData);

request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = data.Length;

    using (var stream = request.GetRequestStream())
    {
        stream.Write(data, 0, data.Length);
    }
var response = (HttpWebResponse)request.GetResponse();

var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();