How to use an externally registered domain name for a DH sub-domain?



I have a website hosted with DH at a sub-domain of my main site:

Now, I have also a German domain name which I had to register with a different company since DH does not host .de domains.

What I would like to set up is that people can browse my DH sub-domain under the name of the German domain. That is, the files that they see are stored here, e.g.:

But what is written in the browser address field is:

I tried to play around with the settings at my German registrar, but no success. (More specifically, it worked for the homepage, but not for any other of the pages - the browser address always switched back to the DH sub-domain.)

So I was wondering whether this can be done from within the DH panel - for instance, by adding as a new domain. (The other registrar allows me to specify the DH nameservers as the nameservers for my domain.)

Thanks a lot for any comments/help!