It's very simple. Just spin up a LAMP app with something.mydomain.com as the location. Then, in the file manager create a file .htaccess in the public directory with the following content:

# Permanent URL redirect RewriteEngine on Redirect 301 / https://externaldomain.com/path

Any requests to something.mydomain.com will then be forwarded to externaldomain.com/path

There is plenty of documentaton on this if you do a Google search for 301 redirect .htaccess. there are also generators that will provide you with the correct code for different scenarios, for example redirecting specific pages.