Information required to publish the MQTT message through the IoT message broker.
The MQTT topic of the message. You can use a string expression that includes variables (`$variable.<variable-name>`) and input values (`$input.<input-name>.<path-to-datum>`) as the topic string.
`$variable.<variable-name>`
`$input.<input-name>.<path-to-datum>`
You can configure the action payload when you publish a message to an IoTCore topic.