When the 3rd party cookies are blocked, the Visitor ID is not being
shared between different domains. I have implemented a custom code to
accomplish the Visitor.appendVisitorIDsTo(URL) solution:
you think that it would be useful to create an extension to accomplish
this in an easy way? Something like Google Tag Manager does for this
scenario that it's a simple checkbox...
If the browser rejects 3rd party cookies and you are not implementing
you will need to decorate every URL that redirects the user from DomainA
someday the decoration of the URL with the user ID will be able to be
automated in an easy way. Hope this helps.