Could you please provide a little more information on what exactly you want to add?

I'd like to add a pop-up modal window once a user clicks a button.

To be a bit more specific:

When a registered logged in user creates a new topic on the forum. Once the user clicks the "Add Topic" button, a modal popup appears to inform the user that the newly created topic is pending moderation. 

Please let me know if you have any ideas as to how I can do this or if there is any code out there that I can past into the functions.php file. 

Thanks!