You specify your server's host name during your very first login to Panel. If you want to change the host name later, you can do it through Panel.
Note: Specifying an invalid host name will result in unpredictable Panel behavior and server malfunction. The host name must resolvable from the Panel-managed server, especially if Customer and Business Manager is installed.
To change your server's host name:
This should be a fully qualified host name, but without an ending dot (for example,