In order to listen for an incoming event, you need to add an event listener for that event name. So in the example below you will see that an event listener was added for dashboardLoaded. 
<!DOCTYPE html>
<html lang="en">
  <head>
    <script>
      const messageListener = (e) => {
        console.log(e.detail);
      };
      window.addEventListener("dashboardLoaded", messageListener);
    </script>
  </head>
  <body>
    <explo-dashboard
      dash-jwt="<your-jwt>"
    />
  </body>
</html>
The console log output looks like this: iFrame output