I'm using namecheap and it supports ddns I can update my @ from one box, but I can update a subdomain.

How do I configure the code to work on a subdomain

This works.

Code:
# /etc/ddclient.conf

protocol=namecheap
server=dynamicdns.park-your-domain.com
login=l...n.us
password=fxxx
@
daemon=3600

I tried this (supposedly) for a sub-domain from another machine

Code:

# /etc/ddclient.conf

protocol=namecheap
server=dynamicdns.park-your-domain.com
login=l...n.us
password=fxxx
subdomain
daemon=3600