The NETBIOS name is created automatically based on the domainname in the installer. It is not possible to edit it, although some older windows environments which you want to takeover are configured with netbios != domainname.
When the installer finishes the base installation in the GTK dialogue and you receive the domain setup part in the browser view (where the hostname, domain, etc. is configured), simply switch to a shell by typing CTRL-ALT-F2.
In this shell you can force the name of the NETBIOS value within the installation envrionment with the help of chroot and the ucr force-scope:
chroot /target /bin/bash
ucr set --force windows/domain="EACH-STRING"
Switch back with CTRL-ALT-F1 and continue.