Want to bind WPF in xaml other than behind class

<Window:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        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:local="clr-namespace:WpfApp1"
        mc —Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0"Name="MyStackPanel">
            <TextBox Text="{Binding SimpleText, UpdateSourceTrigger=PropertyChanged}"/>
            <TextBlock Text="{Binding SimpleText}"/>
        </ StackPanel>
        <StackPanel Grid.Row="1">
            <TextBlock Text="{Binding NotifyText}"/>
        </ StackPanel>
    </Grid>
</Window>

using System.ComponentModel;
using System.Threading;
using System.Windows;

namespace WpfApp1
{
    /// <summary>
    /// MainWindow.xaml Interaction Logic
    /// </summary>
    public partial class MainWindow—Window
    {
        private Timer MyTimer;
        private SimpleModel MySimpleModel=new SimpleModel();
        private NotifyModel MyNotifyModel=newNotifyModel();

        public MainWindow()
        {
            InitializeComponent();

            MySimpleModel.SimpleText="Enter here to rewrite the label.";
            MyStackPanel.DataContext=MySimpleModel;
            DataContext=MyNotifyModel;
            MyTimer = new Timer (Tick, null, 0, 1000);

        }

        private void Tick (object state)
        {
            // ★ SimpleModel does not change when changing values from the source (value changes are not notified to xaml)
            // MySimpleModel.SimpleText=DateTime.Now.ToString("HH:mm:ss");

            // NotifyModel changes the display when you change the value from the source.
            MyNotifyModel.Next();
        }
    }

    public class SimpleModel
    {
        public string SimpleText {get;set;}
    }

    public class NotifyModel:INotifyPropertyChanged
    {
        public string NotifyText
        {
            get {return Text;}
            set
            {
                Text = value;
                OnPropertyChanged (nameof (NotifyText));
            }
        }
        private string text;
        private int Count = 0;

        public void Next()
        {
            Count++;
            Count=(Count>5) ?0:Count;
            NotifyText=new string('■', Count);
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged (string info)
        {
            if(PropertyChanged==null)return;
            PropertyChanged.Invoke (this, new PropertyChangedEventArgs(info));
            // PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(info)); // C#6.0 or later, the above two lines of rewrite (Null condition operator)
        }
    }
}