If this question is unclear or doesn't belong here/is duplicate,let me know.
I have run two servers:one (Apache) on port 80,and second (custom) on port 81.
Now,the custom server is used for some scripting and only listening on /cgi-bin/
Now,i ported my Apache server to a custom url abc.xyz,and added
Redirect "/cgi-bin/" "http://127.0.0.1:81/cgi-bin/"
What you are looking for is not for Apache to serve a redirect, but for Apache to act as a reverse proxy.
ProxyPass "/cgi-bin/" "http://127.0.0.1:81/cgi-bin/" should work for you.