Note: This setting lives in GA4, not GTM. You do not need to make any changes in GTM for this to work.
Go to analytics.google.com and open your GA4 property
Click Admin (bottom left)
Under Data collection and modification, click Data Streams
Select your web data stream
Scroll down to Google tag and click Configure tag settings
Click Configure your domains
Click Add condition and add your own website domain (e.g. myclub.co.uk)
Click Add condition again and add eventrac.co.uk (or your custom Eventrac domain if you use one)
Click Save
What this does:
GA4 will automatically append a _gl parameter to links between your two domains, allowing it to stitch the user's session together. This means source and medium (e.g. "facebook / social" or "google / cpc") will be correctly attributed all the way through to the conversion, rather than showing as "direct" when the user arrives on the Eventrac portal