Default Subdomain

[SergheiH]

Hi

I'm using 1and1 dedicated server with Plesk installed.
Right now if entering on any subdomain of my main domain, like: not-existing-subdomain.domain.com it will show the files from the main domain: www.domain.com

What I need is to show for all not existing subdomains the files from another directory of an existing subdomain, not from here:
/var/www/vhosts/domain.com/httpdocs

but from here:
/var/www/vhosts/domain.com/subdomains/existing-subdomain/httpdocs

A simple redirect won't work because I am redirecting the users to their "own" subdomain: username.domain.com , and they have to remain there.

Thanks