<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">

     <Grid
          MouseLeftButtonDown="MouseLeftDownForGrid1"
          x:Name="MyGrid1"
          Background="#858585">
     </Grid>
    
</Window>

using System.Windows;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Shapes;

namespace WpfApp1
{
     public partial class MainWindow : Window
     {
          private Point _mousePositionOld = new Point(0,0);

          public MainWindow()
          {
               InitializeComponent();
              
               // for binding
               this.DataContext = this;
          }

          public void MouseLeftDownForGrid1(object sender, MouseButtonEventArgs mouseEvent)
          {
               Point mousePosition = mouseEvent.GetPosition(MyGrid1);
               if (_mousePositionOld!=new Point(0,0))
               {
                    // create Line
                    var line = new Line();
                    line.Stroke = new SolidColorBrush(Color.FromRgb(0, 255, 0)); // green color
                    line.X1 = _mousePositionOld.X;
                    line.Y1 = _mousePositionOld.Y;
                    line.X2 = mousePosition.X;
                    line.Y2 = mousePosition.Y;
                    line.HorizontalAlignment = HorizontalAlignment.Left;
                    line.VerticalAlignment = VerticalAlignment.Top;
                    line.StrokeThickness = 3; // line thickness

                    // add line to grid
                    MyGrid1.Children.Add(line);
               }

               _mousePositionOld = mousePosition;
          }
     }
}