How can I add a DNS record so that example.com resolves to my server.example.com in the OS X Server app? The default DNS setup leaves me with a record pointing server.example.com to my IP address. I've added a couple of aliases like www, mail.

But how can I add example.com, essentially the root domain so that when I type example.com in the web browser my web server gives me the page?

For example some DNS providers use a special syntax @.example.com -> server.example.com to specify the requests need to go there.

PS. Server is already configured to use itself as the a DNS.

OS X Server 4
If you have added a machine record (e.g. server.example.com) the primary zone example.com has been already created automagically.

To view all records (including your primary and the reverse zone) click on the gear near the bottom of the window and choose 'View All Records'.