Please note that this guide requires that you are logged into your Plesk account, if you are having trouble doing so please see here.
To add a subdomain to Plesk we would typically recommend you do this directly in Plesk itself under:
Domains → domainname → Subdomains
On our VM plans you will also have to set up the A record for the subdomain in cp.blacknight.com (under Registered Domains → domainname → DNS ) if using our nameservers.
As always, should you require more assistance in relation to this issue, please contact our Helpdesk explaining your issue - and what you have done to this point, and we will be happy to assist you further.
Please see our full contact information, on our main website: Contact Us.
Alternatively, email email@example.com for assistance.