before the class declaration, so it will look like
[ToolboxItemAttribute(true)]
[DefaultProperty("Text"),
ToolboxData("<{0}:MyContainerControl runat=server></{0}:MyContainerControl>")]
public class MyContainerControl : System.Web.UI.HtmlControls.HtmlContainerControl
they probably set it false when they were designing the class because it was not meant to be on the ToolBox