Login tests in .Net website

I have a web application with roles and membership. It works fine. This is an example of the controller:

[InitializeSimpleMembership]
public class MyController : ApiController, IDisposable
{ 
...
    [Authorize(Roles = "Admin")]
    [HttpPost]
    public HttpResponseMessage MyAction(MyViewModel vm)
    {
      //...
    }
}

I wanted to create a test to check the authorization of the methods, but I get an authorization error [HTTP Error 401 Unauthorized] - although I login with the correct credentials and this user has the right role.

   [TestMethod]
    public void MyTest()
    {
        Login("Admin1", "123546", true);   // true/false - remember me
        var json = new { Key1 = "Val1", Key2 = Val2 };
        string methodURL = "My/MyAction";
        string method = "Post";
        JavaScriptSerializer jss = new JavaScriptSerializer();
        string jsonChanged = jss.Serialize(json);
        HttpStatusCode hsc = MyWebRequest(methodURL, method, jsonChanged);
        Assert.AreEqual(HttpStatusCode.OK, hsc);
    }

    private static HttpStatusCode MyWebRequest(string methodURL, string method, string json)
    {
        string URL = "http://localhost/MySite/Action/" + methodURL;
        var httpWebRequest = (HttpWebRequest)WebRequest.Create(URL);
        httpWebRequest.ContentType = "text/json";
        httpWebRequest.Method = method;

        using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
        {
            streamWriter.Write(json);
            streamWriter.Close();
        }

        HttpWebResponse httpResponse;
        try
        {
            httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();   // This is where I get my error
            return httpResponse.StatusCode;
        }
        catch (System.Net.WebException ex)
        {
            return HttpStatusCode.Unauthorized;
        }                   
    }


   private static void Login(string userName, string password, bool remeberMe)
    {
        const string URL = "http://localhost/MySite";
        string Login_URL = "/Account/Login";
        var request = (HttpWebRequest)WebRequest.Create(URL + Login_URL);
        request.ContentType = "application/x-www-form-urlencoded; charset=UTF-8";
        request.Method = "POST";
        ASCIIEncoding encoding = new ASCIIEncoding();
        string stringData = "UserName=" + userName + "&Password=" + password + "&RememberMe=" + remeberMe;
        byte[] data = encoding.GetBytes(stringData);
        request.ContentLength = data.Length;

        Stream newStream = request.GetRequestStream();
        newStream.Write(data, 0, data.Length);
        newStream.Close();
        string res = string.Empty;
        var httpResponse = (HttpWebResponse)request.GetResponse();
        using (var streamReader = new StreamReader(httpResponse.GetResponseStream(),true))
        {
            res = streamReader.ReadToEnd();
        }
    }

What's the right way to test authorizations methods in tests?