Under Admin > Channels > Portals > Edit(corresponding to the portal name), you would have the option to provide the Portal URL. Here, you could provide a vanity URL like help.yourcompany.com (help is the subdomain) and associate it with your Product portal.

Before using this field please ensure that you have created a CNAME record in your DNS Zone file to point help.yourcompany.com to yourcompany.freshdesk.com(your Freshdesk Account URL). Once this is done, you would be able to access the newly created Product Portal using the specified Portal URL.