<Window x: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">
     <StackPanel HorizontalAlignment="Stretch">

          <TextBlock MouseLeftButtonDown="MouseLeftDownForBlock1"
               x:Name="MyTextblock1"
               Text="Good bye"
               Margin="0 200 0 0"
               HorizontalAlignment="Center"/>
         
     </StackPanel>
</Window>

using System.Windows;
using System.Windows.Input;

namespace WpfApp1
{
     public partial class MainWindow : Window
     {
          public MainWindow()
          {
               InitializeComponent();
              
               // for binding
               this.DataContext = this;
          }

          public void MouseLeftDownForBlock1(object sender, MouseButtonEventArgs mouseEvent)
          {
               Point mousePosition = mouseEvent.GetPosition(MyTextblock1);
               var mousePositionScreen = MyTextblock1.PointToScreen(mousePosition);
               Point mousePosition2 = MyTextblock1.PointFromScreen(mousePositionScreen);
          }
     }
}