It would need a teeny little plugin to bind that document.referrer to a Bubble variable. There must be one out there - I did it in one of mine but that plugin is massive overkill for this.
maybe this one?
https://bubble.io/plugin/find-website-referrer-1588802890174x715063361641644000
or a bit more …
🆕 [FREE PLUGIN] Referrer URL Complete