emqx4.3/rel/config/ee-examples/message-transformation.conf.example

message_transformation {
  transformations = [
    {
      name = trans1
      description = ""
      failure_action = ignore
      log_failure {level = warning}
      operations = [
        {
          key = topic
          value = "concat([clientid,'/',topic])"
        }
      ]
      payload_decoder {type = none}
      payload_encoder {type = none}
      topics = [
        "#"
      ]
    }
  ]
}