So that when you try to leave the field, a message pops up saying "This field is required. Please enter a value."?
I found one for normal form fields that works with the "On Blur" action, but it doesn't work on radio buttons or combo boxes.
How is that going to work for radio-buttons, exactly? You want to be able from widget to widget to be able to select one of them...
This is a bad approach in general and is very user un-friendly. You should use other means of informing the user that a value is required, not force them.