ExternalLoginViewModel.cs

using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Windows.Input;
using Windows.Security.Authentication.Web;

namespace UWP_ExternalLogin.ViewModels
{
    public class ExternalViewModel : ViewModelBase
    {
        //private readonly string URLHttps = "https://localhost:44300/";
        private readonly string URLHttps = "https://gf-apiv-1.azurewebsites.net";
        private string AccessToken = string.Empty;
        private CookieContainer cookies = null;

        public ICommand GetExternalLoginsCommand { get; private set; }
        public ICommand GetAccessTokenCommand { get; private set; }

        private string message;
        public string Message
        {
            get { return message; }
            set { Set(ref message, value); }
        }


        public ObservableCollection<ExternalModel> Externals { get; private set; }

        public ExternalViewModel()
        {
            GetExternalLoginsCommand = new RelayCommand(GetExternalLogins);
            GetAccessTokenCommand = new RelayCommand<ExternalModel>(GetAccessToken, (em) => em != null);
            Externals = new ObservableCollection<ExternalModel>();
        }

        private async void GetExternalLogins()
        {
            Externals.Clear();
            cookies = new CookieContainer();
            var handler = new HttpClientHandler() { UseCookies = true, CookieContainer = cookies };

            using (var client = new HttpClient(handler) { BaseAddress = new Uri(URLHttps) })
            {
                var response = await client.GetAsync("/api/Account/ExternalLogins?returnUrl=%2F&generateState=true");
                var json = await response.Content.ReadAsStringAsync();
                var models = JsonConvert.DeserializeObject<List<ExternalModel>>(json);
                foreach (var item in models)
                {
                    Externals.Add(item);
                }
            }

            (GetAccessTokenCommand as RelayCommand).RaiseCanExecuteChanged();
        }

        private async void GetAccessToken(ExternalModel em)
        {
            Message = "Get permissions...";
            var url = string.Format("{0}{1}", URLHttps, em.url);
            var wab = await WebAuthenticationBroker.AuthenticateAsync(
                WebAuthenticationOptions.None,
                new Uri(url),
                new Uri(URLHttps));

            if (wab.ResponseStatus == WebAuthenticationStatus.Success)
            {
                Message = wab.ResponseData;
            }
        }
        
        private async void RegisterUser(string email)
        {
            if (!string.IsNullOrEmpty(AccessToken))
            {
                using (var client = new HttpClient() { BaseAddress = new Uri(URLHttps) })
                {
                    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", AccessToken);
                    var data = new Dictionary<string, string>
                    {
                        { "Email", email }
                    };

                    var response = await client.PostAsync(
                        "api/account/registerExternal",
                        new FormUrlEncodedContent(data));
                    response.EnsureSuccessStatusCode();
                }
            }
        }
    }

    public class ExternalModel
    {
        public string name { get; set; }
        public string url { get; set; }
        public string state { get; set; }
    }
}