code: func (conn *BytesConnectionStr) SendSyncMessage(topic string, key client.Encoder, value client.Encoder) (offset int64, err error) {
  	msg, err := conn.multiplexer.hashSyncProducer.SendMsgToPartition(topic, DefPartition, key, value)
  	if err != nil {
  		return 0, err
  	}
  	return msg.Offset, err
  }
docs: //SendSyncMessage sends a message using the sync API and default partitioner