First you click on "Settings" please and then on "Own Domains":
Then you see the menue for your own domains:
Before you can add a new Subdomain it is mandatory that you add an imprint and privacy-disclaimer.
First the imprint:
Then the privacy-disclaimer:
Now you can add your new subdomain:
Once you have added your subdomain the system will generate a destination domain for you:
You need this destination domain for your settings at your domainhost.