Create a demo Switch in OH2, place it in a sitemap, toggle it and monitor with MQTT.fx the broker to see if the ON/OFF messages arrive to the topic (openhab2/out/Test_MQTT/command)
Switch Test_MQTT "Testing MQTT" <siren> {mqtt=">[broker:openhab2/out/Test_MQTT/command:command:*:default]"}
Check also: MQTT Binding (v1.11) Getting Started 101