Recently one of the websites I work on have added a form to their website that uses a third-party iFrame.
We do have access to the third party code and have added GTM. in GTM I have managed to successfully implement cross domain tracking and trigger an event when confirmation page inside the iFrame is viewed. This data is then sent to GA and triggers a goal completion. I've also excluded the third party domain from the referral list.
This is good but we're now unable to track if the users came from PPC, Organic, Email etc. I'm struggling to find sources online.
Question: How do I track the source of the user on the original domain when triggering an event in a third-party iFrame for analysing goal completions by channel?