You can always open up the SmartBox table for the SmartObject you created and modify the table schema to incorporate a default value there. Keep in mind the caveat to this is you may lose those default values if you repeatedly publish your SmartObject to the SmartBox.

Another option would be to 'hard-code' some kind of value in the artifact calling the SmO event. Are you trying to write this from a custom event, or from inside a K2 process?