Domain Mapping and Child Site

Hi,
I'm having an issue, the DNS settings for our custom external domain are properly configured to point to our server.
Now we need to map the external site to our child site. We added via cPanel our external domain but the question is what to add as doc.root?
Let's suppose that the external domain name is http://www.fantastic.com and child site fantastic.parentsite.com
If we want to be able to manage everything in our hosting including email we need to create an add-on with fantastic.com right?
But in cPanel settings there are few things to add:
New Domain Name: fantastic.com
Document Root: public_html/fantastic.com
So when this is specified and go to fantastic.com after adding fantastic.com as primary domain with the plugin settings, we have a message "Template Error: The template file must be given (or the template could not be opened)" as there's no WordPress files.
URL point to : fantastic.com/cgi-sys/defaultwebpage.cgi
And when the Document Root is: public_html/parentsite.com we have the same error.
So how can we properly configure an add-on?

Thanks for your answer