How to track conversion sources?

Question

Marketing

Greetings,
There is a site, some payment systems are connected on it, not all of which redirect the user to the payment confirmation page. Such conversions are transmitted to Google Analytics via the Measurement Protocol, data on the conversion itself is taken from CRM. However, tracking sources/channels is very important for me, but I don’t understand how to correctly take into account the user’s source/channel and pass it to Google Analytics along with the conversion through the Measurement Protocol? At what stage and where to get it from, how to store/associate correctly, and so on, it is desirable to approach the highest possible threshold of accuracy.
I would be grateful for any answers on the topic!

Answer 1

to start, see and carefully links from there
your task is to learn how to link user cookies and the Measurement Protocol

Answer 2

Pass the visitor's id to crm, it's in the cookie.
Then return back the event with this id.
It is also useful to write it in a custom dimension so that it is possible to make a selection.