If you try to add a CNAME record for the zone apex (root level or @), the following error message will be displayed:
Command failed; error adding record: zone already has records. cannot create CNAME record
This is because CNAME records cannot be added to the root zone. The detailed explanation is described in several RFC documents, namely: RFC 1034 § 3.6.2, RFC 1912 § 2.4 and RFC 2181 § 10.1.
The two most common, easy to implement alternatives are A or AAAA records, or setting up web redirects instead.
A or AAAA Records
By adding an A (IPv4) or AAAA (IPv6) record pointing to the address that the target of the CNAME will ultimately resolve to.
This can be easily done from your Control Panel or via the ModifyDNSZone command.
Web Redirects
By setting up a redirect, also know as web forwarding. This can also be done from your Control Panel or via the AddWebFwd command.
COMMAND = AddWebFwd
SOURCE = http://example.com
TARGET = http://www.other-domain.com
TYPE = RD