Configure apache for WorePress multisite (subdirectory-mode) using custom domain for subpage

I have installed a Multisite-WordPress in “subdirectory-mode” (not subdomain) on a virtual host.
The main page and all subpages work fine using the default urls like:

But now I want to provide one of the subpages within the context of a specific domain (let’s say “”).

I know how to configure WordPress for this scenario, but I cannot get virtualmin/apache to forward (not redirect) request from to so that WordPress knows that the original request comes from (this is important for WordPress-internal redirect to the right subpage).

I’ve tried Domain alias, Proxy Website, Website redirects, … but all without success.
I would be very pleased to get some practical hints from you to reach my goal!

kind regards, Thomas