Hi, I found this description describing LAU keys. IBM Knowledge Center
Given that description, I would propose that you will need to add some sort of custom step which calls a JAR file (hopefully, you have a callable class in the customer infrastructure) that takes the message key from the incoming the request and creates a proper response message key. After creating the key, the service needs to marshal this key to the response. The latter can be done with via DevTest {{propertyName}} notation. Creating the actual LAU response key will require customization as best I can tell.