I found myself in a position.. (explained below). I needed a solution to a client problem, so I built it as an MVP. It's now being used in production by this client, and I'm posted it here for feedback.
It started off as a problem summed up as: "what are the existing cross-domain traffic attribution solutions"
Client X. They have all marketing pages on "root*com"
Their signup flow is on "accounts.root*com"
Their actual SaaS app (which has Mixpanel installed) is on: "app.root*com"
Outline of Problem:
You want to drive traffic to their homepage (rootcom, rootcom/case-studies...)
You want to use UTM variables to track ad performance from Google Ads.
You have to get those UTM variables down into "app.root*com" (and inside mixpanel)- so we know which exact people+purchases are attributed to which campaigns/sets/ads.
I came to find some tech solutions, but like I said.. I wanted to build it in a re-usable way, for other people in this dilemma.
it allows me to drop a tag on "rootcom", use UTM links, then re-call those UTM links from "app.rootcom" when an account is logged-in / purchase is made..