The Storyboard that the BeginStoryboard starts.
This property is read/write. The default value is null.
The XAML syntax for BeginStoryboard and its Storyboard property are examples of XAML content property syntax. In XAML content property syntax, the intervening property element (in this case, BeginStoryBoard.Storyboard) can be omitted, and the property value Storyboard can be a direct object element child of the BeginStoryboard object element in your markup.