How to add a ScrollBar to a Stackpanel

Put it into a ScrollViewer.

Stackpanel doesn't have built in scrolling mechanism but you can always wrap the StackPanel in a ScrollViewer

<ScrollViewer VerticalScrollBarVisibility="Auto">
  <StackPanel ... />
</ScrollViewer>

It works like this:

<ScrollViewer VerticalScrollBarVisibility="Visible" HorizontalScrollBarVisibility="Disabled" Width="340" HorizontalAlignment="Left" Margin="12,0,0,0">
        <StackPanel Name="stackPanel1" Width="311">

        </StackPanel>
</ScrollViewer>

TextBox tb = new TextBox();
tb.TextChanged += new TextChangedEventHandler(TextBox_TextChanged);
stackPanel1.Children.Add(tb);