Block websites using Fortinet-Webfilter-Category-Block attribute freeradius

Hi all, I'm from Openstack Background, I have integrated freeradius server with fortigate, but when I am reading about fortinet radius server I found this attribute Fortinet-Webfilter-Category-Block which mean I can block websites from freeradius using that attribute in reply section but when I applied this attribute it give me error and deny user from access internet. The definition of this attribute tell us that we must use octets value Fortinet-Webfilter-Category-Block 17 octets but if I want to block facebook for example what should I using as value of that attribute ?

Thanks.