Add subdomain to DH account

Is it possible to host a subdomain with DH which has been delegated to DH’s nameservers ?

Using DH’s control panel it does not /seem/ to be possible to add a subdomain.

Take for example fictionaldomain.foofoo - DNS for this domain is provided on a non-DH host. I want to host subdomain beta.fictionaldomain.foofoo with DH… but
I can’t figure out how to get DH’s control panel to accept
the details !