Creo UI Editor - Toolkit - Radio group InMixedState - How to apply this attribute?

Can anyone provide an example of how to apply the InMixedState attribute to a Radio Group? 

I want to create a radio group with two selections AND I do not want a default item preselected. I want to force the user to pick one. I my example image below, item one is selected. I want neither to be selected.

The Creo UI Editor documentation seems to indicate this is possible by setting InMixedState. I just can't figure out how to do it.