How do I allow access to a folder only if the content is embedded on another site?

I have a site on a.example.com which is a web application to which I cannot, for many reasons, upload media files.

I want to be able to upload these files to b.example.com/media/ (an Apache/PHP server) so that one is only able to access the files on the /media/ folder through <img>, <media>, <a> and other elements on pages found on a.example.com.

I've tried an .htaccess inside the /media/ folder with Require host and Require forward-dns a.example.com, but this just denies basically all requests.

What else can I try?

Thank you all in advance for your help & time!