EinmaleinsTrainer/Views/QuizView.xaml.cs

using Avalonia.Controls;
using Avalonia.Markup.Xaml;
using Avalonia.Threading;

namespace EinmaleinsTrainer.Views;

public partial class QuizView : UserControl
{
    public QuizView()
    {
        InitializeComponent();

        this.AttachedToVisualTree += async (_,_) =>
        {
            await Dispatcher.UIThread.InvokeAsync(() => { }, DispatcherPriority.Background);

            var tb = this.FindControl<TextBox>("AnswerTextBox");
            tb?.Focus();
        };
    }

    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
    }
}