Modify /etc/rc.conf and add or change the variable hostname therein:

hostname="myveryownhostname"

I believe this is documented in the manual for the hostname utility.

Modify /etc/rc.conf and add or change the variable hostname therein:

hostname="myveryownhostname"

I believe this is documented in the manual for the hostname utility.

If the variable is not there, the hostname of the system will be Amnesiac.

The variable is also documented in the rc.conf manual:

hostname

(str) The fully qualified domain name (FQDN) of this host on the network. This should almost certainly be set to something meaningful, even if there is no network connection. If dhclient(8) is used to set the hostname via DHCP, this variable should be set to an empty string. If this value remains unset when the system is done booting your console login will display the default hostname of Amnesiac.