Cross Domain Tracking in Google Analytics
GATC trackers only work on main and sub-domains you other domains must be added manually.
For example: You have a School.ca website, with a class registration on WebAdvisor-school.ca
Marketing campaigns will bring users to landing pages but the actual registration happens on Webadvisor website.
To understand a users journey, you need to share sessions across all domains or user from landing pages will be recorded as referral.
Here is a good sample of how to set this up in Google tag manger