Customizing the create/update snippet forms in the Manager (Revolution)

Hello,

I am using Modx Revolution, and I’d like to customize the create and update snippet forms in the Manager.
I’d like to embed BeSpin ( https://bespin.mozillalabs.com/docs/embedding/index.html ) into the snippet textarea.

I looked at these files:
manager/controllers/default/element/snippet/
create.php
update.php

Adding a custom script tag, and a link tag to the head section seems easy enough. I just need to create a plugin that listens for the "OnSnipFormPrerender" event, and calls $modx->regClientStartupScript and $modx->regClientStartupHTMLBlock in both of these files.

However, I’m trying to figure out how to add a custom class and options attribute to the snippet text area.

It currently looks like this:



And I want to make it look like this:



Which code do I edit to make this happen?

Thanks!