!!! This plugin is not ready yet !!!
This add-on was built for me.... and maybe for other brave Home Assistant OS users ;-) So, surely hacky but from the skeleton, so, cannot be all that bad!
You will likely want to have the SSH plugin configured and working in order to place your ddclient.conf file under /share/ddclient (you would have to create thie directory for now).
And it should "just work"!