In the Event Channel set the “Separator Character”. As the description says
The trigger channel payload usually only contains the received MQTT topic value. If you define a separator character, for example ‘#’, the topic and received value will be in the trigger channel payload. For example: my_topic#my_received_value.
For a Nashorn example of this in use see MQTT Event Bus Subscription [3.2.0;3.4.9] and for a GraalVM example see MQTT Event Bus [4.0.0.0;4.9.9.9].