This looks like a neat and useful tip, but it's not clear--at least to me--where I enter the username and password for the DNS service I use.

If my username is an e-mail, do I enter it in <key>UserName</key> or <string>nobody</string>? And where do I enter my account password for the DNS service?

I've been using a launchd file to do this for years. So to answer your question.
Neither, that UserName/nobody is for what local account should be used to execute the command.

for example with dyndns.org you would update the script to:
<string>/usr/bin/curl</string>
<string>--silent</string>
<string>--insecure</string>
<string>-u</string>
<string>[username]:[password]</string>
<string>https://members.dyndns.org/nic/update?hostname=[myhost]</string>