From the section [Captive Portal]->[Authentication], you should click on the button [+] in the Authorized Domains frame. In the form that appears, type the domain name and select the flag “External Kerberos 5 Realm”. Don’t forget to configure the realm in the section [Kerberos 5]->[Realms] or if you prefer enable the DNS discovery of the Kerberos V realms.
Regards
Fulvio