Avalonia/AvaloniaApplication4/ViewModels/MainWindowViewModel.cs

using Avalonia.Controls;
using ReactiveUI;
using AvaloniaApplication4.Views;

namespace AvaloniaApplication4.ViewModels
{
    public class MainWindowViewModel : ViewModelBase
    {
        UserControl us = new Auth();
        public UserControl Us { get => us; set => this.RaiseAndSetIfChanged(ref us, value); }
    }
}