1) You can go to your control panel -> Website Domain Manager and add your Subdomain name as if it's a real domain name.
2) After you add your subdomain name into your site, you wil need to make sure you setup an A-Record in your DNS server. The A-Record should point to your account's IP. You can find your Account's IP in your Control Panel -> DNS Manager.
If you are not sure how to Add a A-Record, please contact the hosting provider that's hosting your MAIN DOMAIN NAME. They are responsible for that, not us.