Since you've already done the leg-work for creating the JMS integration, the rest is just utilizing Activiti's built-in APIs to call the appropriate methods to kick off a process. The API Docs list several methods similar to the one you highlighted:
Me personally, I like to look at Activiti's Tests in Github to see how these things work; but basically what you're going to end up doing is: Create a named message event in the process diagram, then call it as the 'messageName' parameter you were asking about. The User Guide documentation is a pretty solid reference as to how these work and setting them up.
Hope this helps to bridge your knowledge here -JEarles