Assuming you have Marketing Professional (to use the Workflows) you could use your non-required field as part of your trigger e.g. Form Submission is "form name" and the Contact Property "non-required field" is "known".
Assuming you have Marketing Professional (to use the Workflows) you could use your non-required field as part of your trigger e.g. Form Submission is "form name" and the Contact Property "non-required field" is "known".