[Nextgrid]

In this case, on the web, there is already a solution which is the Referer HTTP header (https://en.wikipedia.org/wiki/HTTP_referer) - it allows the destination web server (and no one else) to know the referring page, as long as both are served using the same protocol.

If HTTP referrer isn't an option, a custom query parameter like ?referer=source_site is what I'd consider okay - it looks straightforward so someone can decide to remove it manually if they have a good reason to do so. I would avoid stuff like utm_ parameters as they just mean Google Analytics and not only is this blocked by default by my browser & DNS server it also means you don't care about my privacy and are happy for Google to track me (I never click on links in emails for this reason because they all got some kind of scummy shit to track me that involves a third-party).