Hi Janelle,
There's nothing that would natively tie to the Custom Form, but -- assuming making it a required field would not be an option, given the workflow -- what you could do is to either add an Approval Path to the task in question, or put the custom form on an Issue on the Task.
Either approach would would prevent the Task from being changed to Complete (CPL) status, since it would be "pending Approval" (CPL:A) or "pending Issues" (CPL:I), respectively.
Regards,
Doug