It’s one of the recently added features, see also NethServer project milestone 8.3
On my NethServer VPS where LDAP is used, the field is shown.
Is the latest migration tool (1.1.0) installed?
[root@nethserver ~]# rpm -q nethserver-ns8-migration
nethserver-ns8-migration-1.1.0-1.ns7.x86_64
Yes, the host name or IP address of the NethServer 8 cluster leader node.