public MainPage()
{
    InitializeComponent();
    ListBoxDemo.Loaded += new RoutedEventHandler(ListBoxDemo_Loaded);
}

Step 2: Than create a DependencyProperty called – ListVerticalOffsetProperty, so we can use it later to perform action upon reaching the Last Item in the Listbox.

public readonly DependencyProperty ListVerticalOffsetProperty = DependencyProperty.Register("ListVerticalOffset", 
typeof(double), typeof(MainPage),      new PropertyMetadata(new PropertyChangedCallback(OnListVerticalOffsetChanged)));
 
public double ListVerticalOffset
{     
    get { return (double)this.GetValue(ListVerticalOffsetProperty); } 
    set { this.SetValue(ListVerticalOffsetProperty, value); }
}

Step 3: Use the following method to find the ScrollViewer object

/// <summary>
/// Finding the ScrollViewer
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="root"></param>
/// <returns></returns>
static T FindChildOfType<T>(DependencyObject root) where T : class
{
    var queue = new Queue<DependencyObject>();
    queue.Enqueue(root);
 
    while (queue.Count > 0)
    {
        DependencyObject current = queue.Dequeue();
        for (int i = VisualTreeHelper.GetChildrenCount(current) - 1; 0 <= i; i--)
        {
            var child = VisualTreeHelper.GetChild(current, i);
            var typedChild = child as T;
            if (typedChild != null)
            {
                return typedChild;
            }
            queue.Enqueue(child);
        }
    }
    return null;
}

Step 4: Now we connect all the dots together, getting the FrameworkElement (the ListBox), searching for the ScrollViewer object and register to the VerticalOffset changed event.

void ListBoxDemo_Loaded(object sender, RoutedEventArgs e)
{   

   FrameworkElement element = (FrameworkElement)sender; 
    element.Loaded -= ListBoxDemo_Loaded;    
    scrollViewer = FindChildOfType<ScrollViewer>(element); 
    if (scrollViewer == null)  
   {        
      throw new InvalidOperationException("ScrollViewer not found.");  
   }     

    Binding binding = new Binding();  
   binding.Source = scrollViewer;   
  binding.Path = new PropertyPath("VerticalOffset");
    binding.Mode = BindingMode.OneWay; 
    this.SetBinding(ListVerticalOffsetProperty, binding);
}

Step 5: Implement the OnListVerticalOffsetChanged to check if the ListBox has reached the last item and if so, create new items.

private static void OnListVerticalOffsetChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{     

     MainPage page = obj as MainPage;
     ScrollViewer viewer = page.scrollViewer; 
    //Checks if the Scroll has reached the last item based on the ScrollableHeight 
    bool atBottom = viewer.VerticalOffset >= viewer.ScrollableHeight;  
   if (atBottom)     
     {       
  //ViewModel will be better........  
       int Count = page.ListBoxDemo.Items.Count;    
     int end = Count + 10;     
    for (int i = Count; i < end; i++)     
    {                              

   page.ListBoxDemo.Items.Add("Dummy Item " + i);   
      }   
  }
}