This function is used to send a message to an app (on the same toolbar/engine) using the key the recipient app was previously registered with.Messages can be sent between HTML apps, or from a web page to app.
Notes:
- The registered app should remain open in order to receive messages.
- This function should be used along with the "RegisterForMessaging()" function.
- In order to send a message from a web page, you must include the web page API code on the page that sends the message
