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