Event Application Flow - fields cannot be configured as required

Hello,
I have configured the Event Application Flow and need to set some fields in the flow blocks as required.  I opened a Sandboxe to configure the fields as required with a rule in HCM Experience Design Studio using the "Define Application Flow Required Fields" action.  However, when I create the rule, the event application flow that I have created does not appear in "Application Flow Name" list.  The Application Flow is active and is of type Event. I do not understand why, is there another way to make these fields required?

kind regards
Andrea