Member App - Additional Validation

I have a form that will have a "Company ID" field that needs to be validated against a database of existing company ids. Functionally, it will work similar to the existing email check, though it will come from a separate table in the database.

I've tried adding an onsubmit function to the form template, but it gets stripped when rendered to the page. Alternatively, I could modify the PerchMembers_Members.class.php - but I'm hesitant to do so.

I'm sure I'm missing something obvious. Ideas?

I figured it out. Super simple. Duplicated the members app, added a static function to check the ID and added the helper to the form template. Once I figured out how helpers work in the context of form validation, it was quite straightforward.