You should let the HostNexus team set up subdomains for you, as Plesk has no way of differentiating between subdomains (newsubdomain.domain.com) and new Domains (
www.newdomain.com) under the same account.
The best way to do this is to open a support ticket from the main HostNexus site (
www.hostnexus.com) and let them know:
* What you want the subdomain to be (eg ????.domain.com)
* Where you want the subdomain to point to (eg
www.domain.com/long/url/page.html)
* What type of subdomain you want (eg Frame Forward, where it keeps the subdomain address in the URL of your browser, or Standard Forward, where it just replaces it with the new URL (the one you answered in point two)).
If you set them up yourself in Plesk, it will use up one of your Domains (aka MultiHosts) from the allowance of however many you were allocated in your plan, and it can then be fun and games later on explaining to the HostNexus guys what you've done (as I found out!). When I registered my ticket, the job was done within a few hours, so it's no major inconvenience.