Testing onLaunch event

The service simulator only allows us to configure JSON inputs for simulating intent events. How can I test my Alexa skill against an onLaunch event? In my case, I am programming two ways of using my skill. The first is if it is invoked without any parameters (slot values), in which case the onLaunch event is triggered. In this case, I prompt the user for the individual parameters/slot values and then process the request. The other way is to invoke my skill with the parameters / slot values. In this case, a separate intent is invoked that processes the request and returns the response. I would like to test the scenario when the skill is invoked without any parameters.