IDFA Compare users clicked on my ad link and installed application on AppStore

Hi, i want to compare users clicked on my ad on my website and users that install application from AppStore. So after i can link this installation and current user. It is available and for it IDFA exists as i know.
But how can i do it, how it works?
As i know i cant get IDFA of user in web by JS or something else... So how can i identificate user by IDFA if i cant get it?