Should the referring URL be the LAST page on your website before you redirect to Realex to enter your details? If so, then what if you have dynamic URL?
Yes, the Referring URL is the last page before the redirect - it's the one with the form that POSTs to HPP. It can be dynamic - just let your Realex support contact know which parts of the URL will change and they can use some regular expressions in your configuration.
For example if your referring URL is
http://example.com/user/32478/cart/78927459345/confirm
they can configure
http://example.com/user/.*/cart/.*/confirm