Hi, there is no official support for the Hosttech.at built into Certify The Web but you could optionally sponsor development of a plugin for Posh-ACME, then we could integrate that: Posh-ACME/Posh-ACME at main · rmbolger/Posh-ACME · GitHub
In general we are not adding more DNS APIs directly expect via Posh-ACME, there are many thousands of DNS providers globally and they rarely share the same API (if they offer one at all). SOAP based APIs like this one are also slightly more complicated to build clients for.
As an alternative, consider:
Implement your own create/delete scripts in any scripting language and call those scripts via the Use Custom Script dns provider option: DNS Scripting | Certify The Web Docs
Using a large DNS provider that already has a supported API. e.g. Cloudflare, AWS Route 53, or one that implements RFC2136 for DNS updates.
You’d need to discuss with Ryan on the Posh-ACME project as I can’t speak for him, his main constraints are time and that he has to buy a domain and host it on that DNS provider in order to build and test. If you can get someone to develop the plugin then submit that as a PR (pull request) to their project then that makes it easier.