var tokenUrl = "http://aplicacion/webservices/auth/token/";
HttpClient client = new HttpClient(handler);

var byteArray = Encoding.ASCII.GetBytes("username:password1234");
client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue(
    "Basic",
    Convert.ToBase64String(byteArray));

HttpResponseMessage response = await client.GetAsync(tokenUrl);
HttpContent content = response.Content;

// ... Comprobar el código de estado
Console.WriteLine("Response StatusCode: " + (int)response.StatusCode);

// ... Leer el token (en caso de que esté como texto plano)
string token = await content.ReadAsStringAsync();

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();