How would one go about setting up DNS in Plesk so that all unspecified subdomain references go to the main domain?

That is to say: By default, the DNS template sets up ftp.domain.com, mail.domain.com, and www.domain.com (if the www is checked).

But what if someone enters: http://blah.domain.com/ or anything else in place of "blah". Is there a way to default this to http://domain.com/ ?

I'm new to DNS, so I'm not sure. I put in a CNAME record as:
*.domain.com CNAME domain.com
but I'm not sure if that is the right way to do this.

Thoughts? Suggestions?

Thanks!

-Danimal