I have not done this myself, but I did some digging around and it looks like you would use Apache’s ProxyPass protocol.
So in the example Webflow gave it would look something like this:
ProxyPass "/blog" "http://acme-blog.webflow.io"
This is a simple example. There are additional parameters you can read up on here: mod_proxy - Apache HTTP Server Version 2.4
Also view this page as well: Reverse Proxy Guide - Apache HTTP Server Version 2.4
The section entitled “Simple Reverse Proxying” seems to be particularly helpful
Anyway hope this helps! Let me know if it works I’m kind of curious myself