How can I make it so that a specific answer is required in a form field in order to complete the rest of the survey?

 I would like to require that users have a certain promo code in order to have access to filling out an application. Is this possible?

This is possible using the Conditions feature of the form.

Please check an example idea from this same discussion thread.

How to set an access code to a form

Let us know if you need further assistance.