When using a Custom HTML integration, conversions are not automatically tracked until you add the class om-trigger-conversion to the submit button for your form."
As @dadams suggested, you can add the required class to track form conversions by modifying the Javascript form embed code. Our Custom HTML integration option supports HubSpot’s custom form embed.
The embed code is the only way we support adding a form to a page, and if they allow JavaScript to be used at all, we do have an option in the embed code that would allow you to add a custom class to the submit button: submitButtonClass
If they only allow HTML, that’s not something we’d be able to support with HubSpot forms, as they require the use of JavaScript to support things like smart fields and prepopulating known fields, and it’s also required for lead tracking.