Maybe this link will help:
https://github.com/acmesh-official/acme.sh/wiki/dnsapi2#dns_hetznercloud
Update:
Actually it is not merged. I hope it will be soon. I'm also waiting for this :-)
https://github.com/acmesh-official/acme.sh/pull/6563
Update:
Hacky workaround...
1. Download the file...