There is no restriction on using a Dedicated SSL certificate for a Sub-domain. You just need to include the sub-domain when replying to the SSL Certificate Request email which would be sent by us.
Note: A single Standard SSL purchased, can only be installed on a domain OR a subdomain. If you wish to cover them both using a single certificate, a WildCard SSL must be chosen, as explained below.
While, if you’ve placed an order for the WildCard SSL certificate, the sub-domains would be included by default and you need not specify them separately.