Cross domain tracking allows Google Analytics to track a single user as a continuous session as they cross domains.
From the home dashboard, click on the gear symbol in the lower left corner.
In the Property column, click “Data Streams.” You should see your website listed under stream details.
Click your website. When the sidebar opens up, scroll until you see the Google tags section. Then click “Configure tag settings.”
In the settings section, click “Configure your domains.”
Click “Add condition.” Leave the “Contains” command as it is. You’ll also be prompted to type in a domain.
Once you’ve entered a domain, click the blue save button. You’re good to go with your new cross-domain tracking stream.