Apple Airport Extreme tutorial question

In the tutorial located https://www.dyndns.com/support/kb/apple_airport_with_custom_dns.html

I have a question about the

Blockquote Populate the Hostname field with your full hostname from your Custom DNS service. This would typically be: http://your-computer-name.your-custom-dns-zone.com. Blockquote

How do I set up the "full hostname" (how to I get my computer name into custom dns) The tonidoplug computer that I want to set up has one, my ubuntu desktop computer hosts file has one the damn router wants one? what do I put into the custom dns to make this work?

What kind of record do I create?