Possible to run a chatmail relay with duckdns.org?

With duckdns.org you get several free domains, but you can't edit any DNS fields. The chatmail relay installation script asks me to add some fields which I can't do. What I can do however is have a reverse proxy handle different subdomains instead of adding CNAMEs in DNS. Is this doable with the script?

I hope this makes sense, I'm still very much a newbie at networking 😅