public class LoginController : ApiController
{
    UserEntities db = new UserEntities();

    [HttpPost]
    [ActionName("XAMARIN_REG")]
    // POST: api/Login
    public HttpResponseMessage Xamarin_reg(string username, string password)
    {
        Login login = new Login();
        login.username = username;
        login.password = password;
        db.Logins.Add(login);
        db.SaveChanges();
        return Request.CreateResponse(HttpStatusCode.Accepted, "Successfully Created");
    }
    [HttpGet]
    [ActionName("XAMARIN_Login")]
    // GET: api/Login/5
    public HttpResponseMessage Xamarin_login(string username, string password)
    {
        var user = db.Logins.Where(x => x.username == username && x.password == password).FirstOrDefault();
        if (user == null)
        {
            return Request.CreateResponse(HttpStatusCode.Unauthorized, "Please Enter valid UserName and Password");
        }
        else
        {
            return Request.CreateResponse(HttpStatusCode.Accepted, "Success");
        }
    }

}

public class MainActivity : Activity
    {

    EditText txtusername;
    EditText txtPassword;
    Button btncreate;
    protected override void OnCreate(Bundle savedInstanceState)
    {

        base.OnCreate(savedInstanceState);
        // Create your application here
        SetContentView(Resource.Layout.Main);
        txtusername = FindViewById<EditText>(Resource.Id.txtsaveusername);
        txtPassword = FindViewById<EditText>(Resource.Id.txtsavepassword);
        btncreate = FindViewById<Button>(Resource.Id.btnsavecreate);

        btncreate.Click += Btncreate_Click;
    }
    private async void Btncreate_Click(object sender, EventArgs e)
    {
        Login log = new Login();
        log.username = txtusername.Text;
        log.password = txtPassword.Text;
        HttpClient client = new HttpClient();
        string url = "http://localhost:54445/api/Login/";
        var uri = new Uri(url);
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        HttpResponseMessage response;
        var json = JsonConvert.SerializeObject(log);
        var content = new StringContent(json, Encoding.UTF8, "application/json");
        response = await client.PostAsync(uri, content);
        if (response.StatusCode == System.Net.HttpStatusCode.Accepted)
        {
            var errorMessage1 = response.Content.ReadAsStringAsync().Result.Replace("\", "").Trim(new char[1]
            {
            '"'
            });
            Toast.MakeText(this, errorMessage1, ToastLength.Long).Show();
        }
        else
        {
            var errorMessage1 = response.Content.ReadAsStringAsync().Result.Replace("\", "").Trim(new char[1]
            {
            '"'
            });
            Toast.MakeText(this, errorMessage1, ToastLength.Long).Show();
        }
    }
}

System.Net.Http.HttpRequestException: An error occurred while sending the request