I'm not sure actually. I was given names and values to use for the form fields, as well as two hidden fields, and the following url to use as the form action (altered for anonymity)
altered.WebToLead?encoding=UTF-8
When you submit the form, you're taken to this page but its a blank page - assuming because the form info is being inserted into the lead retrieval system. No actual page exists.
I've confirmed with the client that all is working properly on their end.