Vendor-class-based options in KEA DHCP server

Question

I want to send DHCP option 66 with different values, based on vendor class identifier. I've created client classes in config, but I don't know how to tell the server to use them. This is what I have so far:

{
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "vlan3" ]
    },

    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/var/lib/kea/kea-leases4.csv",
        "lfc-interval": 3600
    },  

    "renew-timer": 15840,
    "rebind-timer": 27720,
    "valid-lifetime": 31680,

    "client-classes": [
        {
            "name": "Cisco",
            "test": "substring(option[vendor-class-identifier].text, 0, 5) == 'Cisco'",
            "only-if-required": true,
            "option-data": [
                {
                    "code": 66,
                    "name": "tftp-server-name",
                    "data": "http://10.0.20.2:8008/phoneprov/$MA.xml"
                }
            ]
        },
        {
            "name": "Grandstream",
            "test": "substring(option[vendor-class-identifier].text, 0, 11) == 'Grandstream'",
            "only-if-required": true,
            "option-data": [
                {
                    "code": 66,
                    "name": "tftp-server-name",
                    "data": "http://10.0.20.2:8008/phoneprov"
                }
            ]
        }
    ],

    "subnet4": [
        {
            "subnet": "10.0.20.0/24",
            "pools": [ { "pool": "10.0.20.100 - 10.0.20.199" } ],
            "authoritative": true,
            "require-client-classes": [ "grandstream", "cisco" ],
            "option-data": [
                {
                    "name": "routers",
                    "data": "10.0.20.2"
                },
                {
                    "name": "domain-name-servers",
                    "data": "1.1.1.1, 8.8.8.8"
                }
            ]
        }
    ]
}
}