But then you have no control measure in most platforms, as you cannot track them?

Attribution is not my area of expertise. But could you not track them to see if they convert? This is possible both on the advertiser side and the platform side, provided the advertiser reports conversions to the platform.

You'd want to flag whether and how you captured them.

If you have offline channels directing you to a website as well, like television or print, it can get messy unless architected properly.