Redirecting and detecting URL keywords

CF:2016 | IIS10

I know this is a little more of an IIS question, but wondering if anybody can advise

I need to put keywords into a URL without using actual variables and then redirect and collect the keyword

Example:

https://SomeWebSite/discount1

The folder discount1 does not exist and that is the keyword I need to detect on a CFM page.

I guess I could set up a redirect on 404 errors, to go to a CFM page, and then use CFM to pick up the refer information and then find the word in the URL.

After typing this I tried it out using a redirect in IIS for 404 errors, setting it to removed with a 302 redirect to an absolute URL, a CFM page. In the CFM page I displayed the HTTP_REFERER only to see that it was blank, so it's not passed on and therefore I can't extract the discount1 at the end of the URL

I'm wondering if anybody knows how to retain the original URL or how I should do it so that it actually works.

Thanks