Is there a method/best practice to adding a “www” prior to my unbounce subdomain URL (www.subdomain.domain.com).

My client currently has their primary site available at www.domain.com, and we’ve published an unbounce page that is available at subdomain.domain.com.

My client is concerned that some users may add “www” in front of the subdomain and receive a DNS error. Is there a way to add a www in front of the subdomain?

Any cname, dns info is appreciated!