Referral Access Question
 

I need a little help, please. I am trying to make it so I can access one site from another, but only that one site can do this.

For example, I want to create a directory in site A, www.siteA.com/secret. I want site B to be able to link to it, but not any other site. I don't want the entire site A protected from open use, just a single directory.

How can I set this up, I believe it's a common .htaccess item, but I can't figure it out.

Any help please?