If I understand correctly, the form is inside an embedded iframe while the Snowplow tracker is initialized on the parent page? Unfortunately, this is not supported – the form needs to be embedded on the same page as the tracker.
You could theoretically inject code into the iframe that initializes the tracker and enable form tracking but that requires that the content in the iframe is from the same domain as the parent page which I assume is not the case since you are loading the form from HubSpot.
Here is a related question where the user wants to track a form within an iframe.