question to external blog adress

hello, I have a question. in the changing blog domain dialog, there stands you have to forward the site to the elten blog server. how to do that? I have my own domain but I only want a supp domain use.
so I want use mydomain.duckdns.org/blog instead of domain.duckdns.org. is that possible? and what is meaned with the forwarding thing.

duckdns has no sub domains, and forward is just that you gotta put the ip of elten server.

well I mean not supp domain I mean domain.duckdns.org slash bla ble. so I registry a new domain for the elten blog to can use it so it helsp nothing to enter my own domain without making anything? confused.