A subdomain is something like this:

something.domain.com

A sub-folder is what you are describing:

domain.com/something

Subdomains are routed based on DNS, you can create one and point it to any IP address. A sub-folder is a folder on the server that DNS has pointed to. You cannot use DNS to point a sub-folder to another server. If this configuration is absolutely vital and a subdomain will not do then one way to achieve this would be to create a droplet running Nginx and then create a reverse_proxy configuration for the folder name so that nginx loads content from another server or site.

If you can work with a subdomain it is just a matter of creating an A record with your DNS provider with the name you want and pointing to your IP address.