This article applies to websites hosted on our newer https://cp.blacknighthosting.com platform.
For how to add extra websites or domains on cp.blacknight.com please see here: https://help.blacknight.com/hc/en-us/articles/212512269-Adding-Subdomains
Log in to your https://cp.blacknighthosting.com account and go to:
Sevices > My Services > Click on your plan (not on the domain name itself) > Subdomains
Once you add the subdomain above you will need to create the DNS record for it. If the domain is using Blacknightdns.com nameservers then that is under:
Domains > Manage DNS > Click the Edit Zone icon (page with a pen) to the right of the domain in question
Add a new A record here, with your subdomain name, pointing to the IP address of your site (most likely the same IP address of www.domainname.com)
Please see here for more information: