I assume that you have the following scenario:
User is on www.domainA.com. He clicks a link to www.domainB.com, where domainB.com is considered as part of the same property as domainA.com. In that case, that link could have the URL "https://www.domainB.com/page?intcid=internal-tracking-code".
If the above assumption is correct, then what you want to do is:
- Have an eVar with "Visit" expiration and "Most Recent" allocation. (You could also use "Linear" allocation if that model makes sense to your business.)
- In your implementation, get the URL parameter "intcid" and set its value to your newly created eVar.
Now, you'll be able to report on traffic that moved from domainA.com to domainB.com by using that internal campaign link.