Hi
since you don't need an email
somebody@www.123.abc.com you don't have to create a new domain for
www.123.abc.com
You'll have to do the following:
1. Create new domain
Server: MyServer
Hostname: 123
Domain: abc.com
IP Address: 192.68...
Create DNS: checked
Create DNS-MX: checked
2. create emails in this domain
Set the forward role for each email in the Advanced Settings
e.g.
someone@somwhere.com
3. create codomain
Hostname: www
Domain: 123.abc.com
Create DNS: checked
Create DNS-MX: unchecked
Forward:
http://www.forwardURL.com
That should do it.
I have an Asterisk rule for my main domain e.g. *.domain.tld CNAME domain.tld which allows me to create as many subdomains as I want without creating new DNS entries all the time.