This is from K9 documentation -
Adding macros into the default values of widget properties
Properties use the Default value when users create new instances of the widget. You can set dynamic default values through macro expressions.
The system resolves the macros according to the following rules:
If the property is available in the widget's configuration dialog (Display field in the editing form is checked), the macro resolves directly in the dialog when adding new instances of the widget.
If the property is NOT visible in the configuration dialog, the system dynamically resolves the macro when displaying the page containing the widget instance.
Adding an old example though valid for K9 too - http://blogs.jeroenfurst.nl/blog/november-2010/tips-and-tricks-macros-in-widgets