Here is a more dynamic example where the responseTopic is set progromatically (in the inject node) and passed into the MQTT node.
[{"id":"28f0a45c19686fd5","type":"inject","z":"4b3f21a3.ba434","name":"demo/response2","props":[{"p":"payload"},{"p":"topic","vt":"str"},{"p":"correlationData","v":"[99,111,114,114,101,108,45,100,97,116,97]","vt":"bin"},{"p":"aa","v":"123","vt":"num"},{"p":"userProperties","v":"{\"number1\":1,\"subobj\":{\"sub1\":123}}","vt":"json"},{"p":"responseTopic","v":"demo/response2","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payloadType":"date","x":2380,"y":820,"wires":[["19787fc720cb68ab"]]},{"id":"19787fc720cb68ab","type":"mqtt out","z":"4b3f21a3.ba434","name":"","topic":"demo/request","qos":"","retain":"","respTopic":"","contentType":"text/plain","userProps":"","correl":"","expiry":"","broker":"dc811dae.05f5","x":2580,"y":820,"wires":[]},{"id":"7bc91b8e43fc5073","type":"mqtt in","z":"4b3f21a3.ba434","name":"","topic":"demo/request","qos":"2","datatype":"auto","broker":"dc811dae.05f5","nl":false,"rap":false,"x":2350,"y":980,"wires":[["68b68b0196181c93"]]},{"id":"97d93f0a378e8811","type":"mqtt out","z":"4b3f21a3.ba434","name":"","topic":"","qos":"","retain":"","respTopic":"","contentType":"","userProps":"","correl":"","expiry":"","broker":"dc811dae.05f5","x":2910,"y":980,"wires":[]},{"id":"1e4ccf4168a1a8dd","type":"mqtt in","z":"4b3f21a3.ba434","name":"","topic":"demo/response1","qos":"2","datatype":"auto","broker":"dc811dae.05f5","nl":false,"rap":false,"x":2760,"y":760,"wires":[["21cf9bced1749aec"]]},{"id":"21cf9bced1749aec","type":"debug","z":"4b3f21a3.ba434","name":"req res pattern","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":2940,"y":760,"wires":[]},{"id":"68b68b0196181c93","type":"change","z":"4b3f21a3.ba434","name":"delete topic & add random payload","rules":[{"t":"delete","p":"topic","pt":"msg"},{"t":"set","p":"payload","pt":"msg","to":"$floor( ($random() * 10) + 1)\t","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":2640,"y":980,"wires":[["97d93f0a378e8811"]]},{"id":"b3fec62de35fb3c9","type":"comment","z":"4b3f21a3.ba434","name":"*************** Response topic demo ************************","info":"","x":2490,"y":680,"wires":[]},{"id":"4b105356a7e74661","type":"comment","z":"4b3f21a3.ba434","name":"send payload to \"demo/request\" expect reply on the responseTopic set in the inject node","info":"","x":2580,"y":720,"wires":[]},{"id":"4093181e7772c1ad","type":"comment","z":"4b3f21a3.ba434","name":"fake external application recieving command and responding to reply topic","info":"","x":2540,"y":940,"wires":[]},{"id":"44fe4ed1ec155c5f","type":"inject","z":"4b3f21a3.ba434","name":"demo/response3","props":[{"p":"payload"},{"p":"topic","vt":"str"},{"p":"correlationData","v":"correl sent in msg as a string","vt":"str"},{"p":"aa","v":"123","vt":"num"},{"p":"userProperties","v":"{\"number\":1}","vt":"json"},{"p":"responseTopic","v":"demo/response3","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payloadType":"date","x":2380,"y":860,"wires":[["19787fc720cb68ab"]]},{"id":"8b570801642403a4","type":"inject","z":"4b3f21a3.ba434","name":"demo/response1","props":[{"p":"payload"},{"p":"topic","vt":"str"},{"p":"correlationData","v":"correl sent in msg as a string","vt":"str"},{"p":"aa","v":"123","vt":"num"},{"p":"userProperties","v":"{\"number2\":2, \"number1\":1}","vt":"json"},{"p":"responseTopic","v":"demo/response1","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payloadType":"date","x":2380,"y":780,"wires":[["19787fc720cb68ab"]]},{"id":"4fb4c56e810f05d2","type":"mqtt in","z":"4b3f21a3.ba434","name":"","topic":"demo/response2","qos":"2","datatype":"auto","broker":"dc811dae.05f5","nl":false,"rap":false,"x":2760,"y":820,"wires":[["fc3420b94da5a25d"]]},{"id":"fc3420b94da5a25d","type":"debug","z":"4b3f21a3.ba434","name":"req res pattern","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":2940,"y":820,"wires":[]},{"id":"7b2c384ce917327f","type":"mqtt in","z":"4b3f21a3.ba434","name":"","topic":"demo/response3","qos":"2","datatype":"auto","broker":"dc811dae.05f5","nl":false,"rap":false,"x":2760,"y":880,"wires":[["7cf85a9adfa8092a"]]},{"id":"7cf85a9adfa8092a","type":"debug","z":"4b3f21a3.ba434","name":"req res pattern","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":2940,"y":880,"wires":[]},{"id":"dc811dae.05f5","type":"mqtt-broker","name":"","broker":"broker.emqx.io","port":"1883","clientid":"","usetls":false,"compatmode":false,"protocolVersion":"5","keepalive":"50","cleansession":true,"birthTopic":"","birthQos":"0","birthPayload":"","birthMsg":{},"closeTopic":"","closeQos":"0","closePayload":"","closeMsg":{},"willTopic":"","willQos":"0","willPayload":"","willMsg":{},"sessionExpiry":""}]
The settable inputs are listed in the built in help...
