There is a package available for duckdns, so custom configuration could be avoided.
You don't need to use https in the update_url, even if you intend using https.
Even if IP autodetection is required, you need to add to the URL &ip= without a value according to their instructions and examples.
There is no need to add .duckdns.org in the domain field.
If you use use_https you usually need to add a path to ca-certificates.
Make sure you have curl installed.
My personal recommendation is to avoid using duckdns. A few months ago I've seen a lot of regular timeouts for DNS lookups against their domain.