It is not possible to assign an FQDN hostname to a container through Parallels Management Console; it gives the following error:
In addition, to change the hostname to a 15-character-limited NetBIOS name, a container reboot is required.
Parallels Management Console does not support managing FQDN hostnames for containers and is no longer developed in favor of Virtual Automation.
Use Virtual Automation to be able to edit FQDN hostnames of containers.
To change the hostname of a container to a 15-character-limited hostname, you can use the command line interface:
vzctl set <CTID> --save --hostname test --searchdomain example.com