How can I point an external domain to a subdomain?

Hi,

I have the domain custom.com which I am trying to point to site.example.com which in turn points to example.com/site.

I currently have this in my Virtual Host:

ServerAlias *
VirtualDocumentRoot /var/www/html/%1

I have tried creating a CNAME record for custom.com which points to site.example.com, however for some reason it points to the root site example.com.

How could I allow users to directly point custom domains to subdomains of my site? What would have to be done to my Virtual Host/which records would be required?

Subdomains actually work as intended to since they point to the directories containing the same name. However, any custom domains pointed to any subdomain just points to the root site rather than pointing to the subdomain.

I would appreciate any help with this.

Thank you!