Create www. domain and www.subdomain

Hi,

I have domain xyz.com added to my virtualmin. This automatically added a www.xyz.com alias.

Now what I was is to remove that www.xyz.com alias and I want to either add a www.xyz.com virtual server separately or www.xyz.com sub-server for xyz.com but Virtualmin will not allow me to do that: Failed to create virtual server : The domain name does not need to have www at the start

I want my xyz.com and www.xyz.com to have separate public_html directories.

Can someone please let me know how do I achieve this? Thanks