As an example, if you own the domain 'xyz.com' then you can have the portal URL as 'support.xyz.com', 'sales.xyz.com' etc. The same needs to be created as a CNAME pointing to 'yourcompanyname.freshdesk.com' in your DNS. You will need to create a TXT record in your DNS as well that would be similar to 'fdkey.support.xyz.com' or 'fdkey.sales.xyz.com' pointing to the values displayed under Admin--> Multiple Products--> Settings.