Could you expand on this? I'm having a similar issue where we have leads coming in via two LaunchPoint partners, and I'm struggling to find a way to distinguish between the two. How would I customize the API to populate a field that isn't on the form?
Add an "API partner" field and ask the 2 launchpoint partners to populate this field with a value that is different for each of them and will enable you to know which one did create the record.
Or, for even better tracking, work more closely with the partners to not just insert leads into Marketo. Get them to direct those leads into specific Marketo programs/activities so that they can be assigned proper leads sources, added to nurture, sync'd to SFDC campaigns, etc. I personally would find it a bit alarming to have raw lead records inserted from an API without more control over how they are processed and tracked.