AutomationProperties.LiveSetting not working in WP

I have a TextBlock and I want to track that control from Screen reader and whenever a new value is set to the control in code, the screen reader should readout the new text. This is available in WPF from .NET framework 4.7.1 which is mentioned in the MSDN LINK.

But I am always getting null for the AutomationPeer value. What am I missing in the code? Am I doing it in the right way? Please help.

XMAL

      <Window x:Class="WPFAccessibility.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:WPFAccessibility"
                mc:Ignorable="d"
                Title="WPFAccessibility" Height="450" Width="800">
            <Grid>

                <TextBlock Name="MyTextBlock" AutomationProperties.LiveSetting="Assertive">My initial text</TextBlock>

                <Button Name="Save" Content="Save" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75" Margin="50,321,0,0" Height="49" Click="Save_Click"/>   

            </Grid>
        </Window>

Code

 private void Save_Click(object sender, RoutedEventArgs e)
        {
            // Setting the MyTextBlock text to some other value and screen 
            // reader should notify to the user
            MyTextBlock.Text = "My changed text";
            var peer = UIElementAutomationPeer.FromElement(MyTextBlock); 
           // I am always getting peer value null 
            peer.RaiseAutomationEvent(AutomationEvents.LiveRegionChanged);
        }

Use the CreatePeerForElement method to create a UIElementAutomationPeer for the TextBlock:

private void Save_Click(object sender, RoutedEventArgs e)
{
    MyTextBlock.Text = "My changed text";
    var peer = UIElementAutomationPeer.FromElement(MyTextBlock);
    if (peer == null)
        peer = UIElementAutomationPeer.CreatePeerForElement(MyTextBlock);
    peer.RaiseAutomationEvent(AutomationEvents.LiveRegionChanged);
}