Currenty, the Let’s Encrypt add-on does not support DNS challenge with all-inkl (and their KAS administrative backend). It’s done with their API
Is it possible to add support for it?
Here is the documentation for the acme client: all-inkl :: Let’s Encrypt client and ACME library written in Go.
API documentation is linked at the bottom of the page.
Here is the source code in acme.sh