VkAvaloniaAuth

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Reactive;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using ReactiveUI;


namespace VKGroupPostsView.ViewModels
{
   public class VkLoginViewModel:ViewModelBase
    {
        private bool _LoginPanelVisible = true;
        private bool _TwoFactorCodePanelVisible = false;
        public ulong AppID = 6121396;
        public string LoginText { get; set; }
        public string PasswordText { get; set; }
        public string? TwoAuthCodeText { get; set; } = null;
        public bool TwoFactorCodePanelVisible {
            get => _TwoFactorCodePanelVisible;
            set
            {
                this.RaiseAndSetIfChanged(ref _TwoFactorCodePanelVisible, value);
            }
        }
        public bool LoginPanelVisible {
            get => _LoginPanelVisible;
            set
            {
                this.RaiseAndSetIfChanged(ref _LoginPanelVisible, value);
            }
        }
        public bool SendCode = false;

        public ReactiveCommand<Unit, Unit> AuthCommand { get; }
        public ReactiveCommand<Unit, Unit> SendCodeCommand { get; }

        public VkLoginViewModel()
        {
              AuthCommand = ReactiveCommand.Create(()=>
              {
                  Auth();
              });
              SendCodeCommand = ReactiveCommand.Create(() =>
              {
                    SendCode = true;
              });
        }
        public async void Auth()
        {
            VkNet.VkApi vkApi = new VkNet.VkApi();
          var auth=  vkApi.AuthorizeAsync(new VkNet.Model.ApiAuthParams()
            {
                ApplicationId = AppID,
                Login = LoginText,
                Password = PasswordText,

                TwoFactorSupported = true,
                TwoFactorAuthorization = new Func<string?>(() =>
                {
                    LoginPanelVisible = false;
                    TwoFactorCodePanelVisible = true;
                    Thread.Sleep(300);

                    if(SendCode&TwoAuthCodeText!=null)
                    {
                        string? code = TwoAuthCodeText;
                        TwoAuthCodeText=null;
                        SendCode = false;
                        return code;
                    }
                      return "";
                })
            }).GetAwaiter();
            auth.OnCompleted(() =>
            {
                try
                {
                    auth.GetResult();
                    Debug.WriteLine("AUTH OK");
                 }
                catch (Exception ex) {
                    Debug.WriteLine(ex.Message);
                }

            });


        }
    }
}


Window

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
               xmlns:vm="using:VKGroupPostsView.ViewModels"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:Class="VKGroupPostsView.Views.VkLoginView">
    <UserControl.DataContext>
        <vm:VkLoginViewModel></vm:VkLoginViewModel>
    </UserControl.DataContext>
    <Grid>
        <TextBlock Margin="10"  Text="Авторизация" HorizontalAlignment="Center"></TextBlock>
        <StackPanel IsVisible="{Binding  LoginPanelVisible}" Margin="50">
            <TextBlock HorizontalAlignment="Center"  Text="Логин"></TextBlock>
            <TextBox  Margin="0,20,0,0" Text="{Binding LoginText}"></TextBox>
            <TextBlock HorizontalAlignment="Center" Margin="0,20,0,0" Text="Пароль"></TextBlock>
            <TextBox Margin="0,20,0,0" Text="{Binding PasswordText}"></TextBox>
            <Button Command="{Binding AuthCommand}" HorizontalAlignment="Center" Margin="20"> Войти</Button>
        </StackPanel>
        <StackPanel IsVisible="{ Binding TwoFactorCodePanelVisible}" Margin="50">
            <TextBlock HorizontalAlignment="Center"  Text="Введите код авторизации"></TextBlock>
            <TextBox  Margin="0,20,0,0" Text="{Binding TwoAuthCodeText}"></TextBox>
            <Button Command="{Binding SendCodeCommand}" HorizontalAlignment="Center" Margin="20"> отправить</Button>
        </StackPanel>
    </Grid>
</UserControl>