Thanks for your reply.
No, it's not a template issue. I'm talking about the PHP which outputs the fields. That happens in sapphire/forms/FormField.php. What I am trying to do here is *replace* the id attribute of each of the fields with the CMS user input. Right now, it uses function id() (line 106 of FormField class) where it outputs the id of the field dynamically. I want it to use my newly created custom field but can't figure out how.