Gets a command to create a header sub-document (if it is not yet created) and set it as an active sub-document instead of the main sub-document.
readonly insertHeader: InsertHeaderCommand
A InsertHeaderCommand object that provides methods for executing the command and checking its state.
You can invoke this command by calling the execute method.
The execute method checks the command state (obtained using the getState method) before executing, and decides whether the action can be performed. The execute and getState methods are members of the InsertHeaderCommand class.
The active sub-document is available via the RichEditDocument.activeSubDocument property.