Ok so it sounds like you have already figured out how to add content to the composer? (If not, referencing this example may help GitHub - discourse/discourse-gifs - this theme component opens a modal when the button is clicked from the toolbar, and then inserts content into the composer based on clicking some content in the modal).
If you’re trying to get a script to load and do something to the post content, I think this post might get you pointed in the right direction… using loadScript and decorateCooked: Difficulties in correctly adding external JavaScript - #4 by StevenTammen