A common support issue is that traffic accross www. and a tickets. don't get counted as the same session causing confusing self-referral traffic to be reported.
There are two main requirements to get this to work:
- Both www. and tickets. must be running analytics.js, not the older ga.js
- Referral exclusions need to be setup for any subdomains that should be excluded as referrals. See: https://support.google.com/analytics/answer/2795830
Aside from the analytics.js upgrade this can be handled by the client through their analytics control panel
Where is gets more complicated is if the website uses an iframe for ticketing. Iframe tracking can be complex depending on where the iframe is coming from. You may by able to use the Linker plugin here: https://developers.google.com/analytics/devguides/collection/analyticsjs/linker but make sure you've ruled out the steps above before looking to this.