with the [Pango text markup language][PangoMarkupFormat]. Also see [gtk.tooltip.Tooltip.setMarkup].
This is a convenience property which will take care of getting the tooltip shown if the given string is not null. #GtkStatusIcon:has-tooltip will automatically be set to true and the default handler for the #GtkStatusIcon::query-tooltip signal will take care of displaying the tooltip.
On some platforms, embedded markup will be ignored.