CayenneLPP codec downlinks

I have a class C node that is successfully sending uplinks using the CayenneLPP lib and codec on the application server. I now want to send a downlink message to device but cannot find any documentation for the format of the downlink JSON when using this codec.

I’ve found the downlink message format in the Cayenne docs but need to know the JSON format the AS needs when sending downlink via MQTT. Also I’m assuming the codec does the base64 encoding - is this a correct assumption?

Hope this makes sense.