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();

HttpClient client = new HttpClient();
client.BaseAddress = new Uri(siteUrl);
client.DefaultRequestHeaders.Accept.Clear();

 HttpContent requestContent = new StringContent("grant_type=password&username=" + Username + "&password=" + Password, Encoding.UTF8, "application/x-www-form-urlencoded");

 HttpResponseMessage responseMessage = await client.PostAsync("Token", requestContent);

// Estos valores los obtienes usando input del usuario
var username = "john";
var password = "pass";

// Estos valores debes conocerlos de antemano para autenticar tu cliente
var clientId = "23343";
var clientSecret = "kjdhf343242j342";

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

var postData = "grant_type=password&username=" + username + "&password=" + password + "&client_id=" + clientId + "&client_secret=" + clientSecret;

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

request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded;charset=utf8";
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();