[HttpPost]
    public ActionResult Login(LoginViewModel viewModel)
    {
        if (viewModel.Username == "alex" && viewModel.Password == "password")
        {
            FormsAuthentication.SetAuthCookie(viewModel.Username, false);
            return RedirectToAction("Index", "Home");
        }

        return View();
    }

private static void GetPerson()
    {
        Console.WriteLine("Username:");
        string username = Console.ReadLine();

        Console.WriteLine("Password:");
        string password = Console.ReadLine();

        using (HttpClient httpClient = new HttpClient())
        {
            HttpRequestMessage authRequest = new HttpRequestMessage();
            authRequest.Method = HttpMethod.Post;
            authRequest.RequestUri = new Uri(@"http://localhost:4391/Account/Login");
            authRequest.Content = new FormUrlEncodedContent(new List<KeyValuePair<string, string>>
            {
                new KeyValuePair<string, string>("Username", username),
                new KeyValuePair<string, string>("Password", password)
            });

            HttpResponseMessage authResponse = httpClient.SendAsync(authRequest).Result;
            IEnumerable<string> cookieValues;
            authResponse.Headers.TryGetValues("Set-Cookie", out cookieValues);

        }
    }

HTTP/1.1 302 Found
Cache-Control: private
Content-Type: text/html; charset=utf-8
Location: /
Server: Microsoft-IIS/8.0
X-AspNetMvc-Version: 5.1
X-AspNet-Version: 4.0.30319
Set-Cookie:   .ASPXAUTH=A6A22208AECA1F3E25B43C834BE058A5019F3A9C55AED099FFAD5B0FE7B289EC2C3F87C157B0C1ED338D1DF0A6469E6C5DE8D9DB7A99D54D992EA10F26424BA579C262B7CD247CA4193879E058A233B7A0BC98E10503440B79EB988239C43696; path=/; HttpOnly
X-SourceFiles: =?UTF-8?B?YzpcdXNlcnNcYWxleGFuZGVyXGRvY3VtZW50c1x2aXN1YWwgc3R1ZGlvIDIwMTNcUHJvamVjdHNcRm9ybVZhbHVlc0F1dGhXZWJBUElcRm9ybVZhbHVlc0F1dGhXZWJBUElcQWNjb3VudFxMb2dpbg==?=
X-Powered-By: ASP.NET
Date: Sat, 19 Apr 2014 20:09:23 GMT
Content-Length: 428

private static void GetPerson()
    {
        Console.WriteLine("Username:");
        string username = Console.ReadLine();

        Console.WriteLine("Password:");
        string password = Console.ReadLine();

        HttpClient httpClient = new HttpClient();

        HttpRequestMessage authRequest = new HttpRequestMessage();
        authRequest.Method = HttpMethod.Post;
        authRequest.RequestUri = new Uri(@"http://localhost:4391/Account/Login");
        authRequest.Content = new FormUrlEncodedContent(new List<KeyValuePair<string, string>>
            {
                new KeyValuePair<string, string>("Username", username),
                new KeyValuePair<string, string>("Password", password)
            });

        HttpResponseMessage authResponse = httpClient.SendAsync(authRequest).Result;

        HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, @"http://localhost:4391/api/Person/1");
        request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        HttpResponseMessage response = httpClient.SendAsync(request).Result;

        if (!response.IsSuccessStatusCode)
        {
            Console.WriteLine("Username or password is incorrect");
            return;
        }

        response.Content.ReadAsAsync<Person>().ContinueWith((x) => {
                Person person = x.Result;

                Console.WriteLine("First name: {0}", person.FirstName);
                Console.WriteLine("Last name: {0}", person.LastName);
            });
    }

[HttpPost]
    public ActionResult Login(string username, string password)
    {
        if (username == password)
        {
            FormsAuthentication.SetAuthCookie(username, true);
            return new HttpStatusCodeResult(HttpStatusCode.OK);
        }
        return new HttpUnauthorizedResult();
    }

HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, authTicket);
Response.Cookies.Add(cookie);