Hello,
If I understood you correctly, the problem is that your client hasn't registered a domain yet, but he wants to see his website at your subdomain? If yes, there is such a useful thing as a tecnical domain. Here is
documentation.
Or you can enable the "Different owners for domains and subdomains" option in the "Policy" module(ISPmanager Business) or add the InsecureDomain Option to the ispmgr.conf file (Lite), and add your subdomain to the alias of your user's web-domain.