Is it possible to output a formated string to kafka

[lexxxel]

I have the following issue, I collect data from a service and parse it via fluentbit and the lua filter. I do have my structured data and I want to send it to kafka. Kafka on the other hand wants to get the message in a very specific format, basically as a raw string, not as json.
For example, if I have the following structured data:
{"a": 3, "b": 4, "c": 8}
My lua script would convert it to:
{"message": "a=3&b=4&c=8"}
And I want to only send the value of message to kafka. Is this possible?
If not, Is it possible at all to only output the value of message?