Default Designer XAML for CustomControl

Very simple question. Been trying to figure it out all morning.

How do you define the default XAML when a UserControl is dropped onto the Designer Surface.  For example, when a TabControl is dropped you get:

<TabControl HorizontalAlignment="Left" Height="100" Margin="128,261,0,0" VerticalAlignment="Top" Width="100"><TabItem Header="TabItem"><Grid Background="#FFE5E5E5"/></TabItem><TabItem Header="TabItem"><Grid Background="#FFE5E5E5"/></TabItem></TabControl>

I'm designing a TabControl-like control, and would like to provide default content as above.

Steve