How do I add my plugin's text to the actual message?

I just made a plugin that allows for users to insert multiple objects into their message. I have the html and javascript for it all worked out and tested. I just don't know how to take my text that it generates and put it into the actual message when I press the okay button.

Anywhere in the code I can find this? It doesn't seem to be in the code for the regular checkbox code, for example.

Thanks!