Hi @Rhodri2
Do you have any query parameters appended to your ad click which could be used instead?
Background: nowadays, many websites do not longer send a complete referrer URL anymore but mostly just the domain information. So, should you know that a certain set of query parameters appended to your website URL indicates a specific ad click, that might be the best choice.
For that, you can leverage the Site Pages > Current Page > Query information.

Else, I would look at the referrer information on your website whether the ad that brought the user to your website contains the information you are looking at
document.referrer
Should this contain the needed information, try Site Pages > HTTP Headers

Cheers from Switzerland!