The task is to send message with concrete value to Kafka topic.
For example, I want to send "someValue" as plain text.
What Kafka property in PublishKafkaRecord_2_6 should I use for this purporse?
I see, for example, "FreeFormTextRecordSetWriter" as a service, which writes the contents of a RecordSet as free-form text.
But where inside PublishKafkaRecord_2_6 I can set my "someValue" to publish it in target topic.
where inside PublishKafkaRecord_2_6 I can set my "someValue"
You wouldn't. The Kafka processor just sets up the producer for Kafka client details and serialization.
You'd setup an incoming processor, such as using GenerateFlowFile