Get referrer string for Universal Link?

You should be able to use NSUserActivity > referrerURL instance property. See https://developer.apple.com/documentation/foundation/nsuseractivity/2875762-referrerurl.

No, there is no way to get the referrer. An Apple employee made that pretty clear on the developer forums: https://forums.developer.apple.com/thread/65423

They say that the only way to get some kind of referrer would be to append it to the URL. Maybe take a look Google's campaign tracking URLs (utm_...).