问如何使用MVVM正确绑定我的文本框

EN

using CSMS_MVVM.ViewModels;
private LrViewModel _lrViewModel;
public LoginRegister()
        {
            InitializeComponent();
        }
private void Page_Loaded_1(object sender, RoutedEventArgs e)
        {
            _lrViewModel = new LrViewModel();
            this.DataContext = _lrViewModel;
        }

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<TextBox Name="regID" Text="{Binding RegId,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Padding="8" Margin="15,110,20,0" VerticalAlignment="Top" BorderBrush="#FFD4F1FF" FontSize="15" ToolTip="Username or Employee ID" FontFamily="Calibri" />
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using CSMS_MVVM.Models;

namespace CSMS_MVVM.ViewModels
{
    class LrViewModel
    {
        public LrModel LrModel { get; set; }

        public LrViewModel()
        {
            LrModel=new LrModel
            {
                RegId = "Value"
            };
        }
    }
}

namespace CSMS_MVVM.Models
{
    class LrModel : INotifyPropertyChanged
    {
        private String _regId;

        public String RegId
        {
            get { return _regId; }
            set
            {
                _regId = value;
                OnPropertyChanged(new PropertyChangedEventArgs("RegId"));
            }
        }

        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        private void OnPropertyChanged(PropertyChangedEventArgs propertyChangedEventArgs)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, propertyChangedEventArgs);
            MessageBox.Show("Here");
        }

        #endregion

    }
}