In your project, create a new file by clicking the green + button. Thimble will automatically give it the name script-1.js.
Thimble will add a bit of code to this new file, but let's delete that for now to keep things neat.
Also, we need to tell the browser to read it. Unlike CSS, we generally do this from the bottom of the HTML page; this is so browsers can read the HTML and CSS first, which means the app will load faster: