By "force" do you mean that you want to convert the user's input to
upper-case, or not allow them to enter anything that is not in upper-case?
Conversion. The form is to be printed, and posted (very retro, I know), but if it is not completed in capital letters it becomes invalid. So, I think conversion would be the most universal method.
Put this as the field's custom validation script:
if (event.value!=null) event.value = event.value.toUpperCase();
Works perfectly. Thank you very much