In Plesk:

Home>>Domains>>(select domain of master site)>> Domain Aliases – put in the name of the new joomla site

Log into the Joomla Master Site Admin >> Components >> Multi Sites >> Manage Sites >> New

Fill out Site Details  property sheet and Save

Goto Joomla install page via the alias created for the master Domain

Follow prompts until database page. Enter info for Master database. Important! click the Advanced button and change from the default prefix jom_ to another name.