When enabling a web site, I will usually dig out the minimum required URL's incrementally, with a combination of rule traces and the browsers developer tools (F12).
One possible methodology would be to try to leverage a Referer value.
For example, something like URL.Host portion of Header.Request.Get("Referer") equals "base website name" then permit the connection.
You'd have to manipulate the referer value to get the base URL.Host value from the referer. You could do that with a regex to pull out the hostname from the complete referer value:
MWG format: regex(^https?://(.*?)/(.*),"\1")
So then you could do something like String.ReplaceAllMatches(Header.Request.Get("Referer"),regex(^https?://(.*?)/(. *),"\1") is in list Base Websites.
For example, if you put community.mcafee.com in the "base websites" list you're comparing against and you have the refer value of https://community.mcafee.com/community/business/email_web/webgateway/content?fil terID=contentstatus[published]~objecttype~objecttype[thread], access to https://img.en25.com/i/elqCfg.min.js could be permitted based on the fact that the regex will return community.mcafee.com with the syntax referenced above.