How to add a subdomain to another server/service?
1. Login to your cpanel hosting account at
https://yourdomain.com/cpanel and go to Zone Editor.
2. Find the domain name you want to edit and then click on the A Record button to the right of the listed domain.
4. Type in the subdomain you want and the IP address of the server you want it to point to, click add an A record.
It should be ready to go but to see if it works try going to the new subdomain or run a ping test to see if you get an IP address back.