"How do a create a subdomain to point to a external web page?"
I've read this a number of times and didn't know what you were trying to ask. But I've just thought you might be asking:
"How can a subdomain take a visitor to an external web page? i.e. www.external.mywebsite.com takes a visitor to www.example.com"
I would do this using a .htaccess file with a 301 redirect.
Let us know if this is what you meant or if you are trying to do something else?