ビューモデルをプロジェクトに追加

MainViewModel.cs

using System.ComponentModel;

namespace MauiApp1 {
    internal class MainViewModel : INotifyPropertyChanged {

        public event PropertyChangedEventHandler PropertyChanged;

        #region プロパティ
        private string _CounterText;
        public string CounterText {
            get => _CounterText;
            private set {
                if (_CounterText == value) return;
                _CounterText = value;
                OnPropertyChanged(nameof(CounterText));
            }
        }
        #endregion

        protected virtual void OnPropertyChanged(string propertyName) {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}