How to get URL of the domain which displaying my web page as iFrame

Hi everyone! my clients use my service as an iFrame, I want to track the domain name or URL of my clients who display my web page as iframe, I am using visibility event, it is working fine but {{PageURL}}, {{PageOrigin}} etc. the variable giving the domain name or URL of my website instead of clients site who are displaying my web page as an Iframe.

Can anyone please guide me, how I can get it?? BTW I am using tag manager to track iframe visibility event.

Thanks in advance!
@Lukas @PhilipF