Sending downlink through MQTT Protobuf?

I’m trying to send a downlink to a node using mqtt with the protobuf marshaler to no success, i can send the hex through queue under the node settings section successfully however. The only “guide” i can find is the gw.proto on github that’s abit too advanced for me to decipher.
Can someone point me in the right direction?

Chirpstack v4
Marshaler protobuf