setting up a user to have a default domain when account created

[Fiberoptck]

I run a small web design consulting firm and we currently use Cpanel/WHM for the hosting of the sites backed with WHMCS as the billing side. Some customers have domains but do not wish to import them with out a finished product done. I came up with the concept of having a default domain address to the user when they pay and sign up on whmcs (ex: 18.a.companydomain.me) or something to this effect so that there is a test domain available to be used. I not sure if this is something I need to do on the WHMCS side (which I will then take my question to them) or if this knowledgeable group might know if there is either a plugin available that applies this or if I am going to have to try to make a script that pushes that create default domain to the account and adds it to the whmcs account as the domain.

I tried googling the question but I do not think I am using the right search terms because I am not getting anywhere with this. Any help would be gladly appreciated!

Thank you!