How do I track page visits for a lead created through an API call?

We use drupal forms on our site to capture leads that make API calls to create the leads in Marketo. We want to be able to track the initial landing page of the lead, but because the leads are being captured via API it looks like original referrer and other tracking information aren't being attributed the leads even though we have Munchkin code on the site.

Is there an easy way to merge the anonymous lead data that Marketo has with the lead the API call creates? Or is there another, easier solution to get the data I want? Any advice is much appreciated.