1. Create new admin.js file in `src/assets/js` directory and add your code to it. If you're going to use jquery, make sure to add `import $ from 'jquery';` at the top of the file.
2. In `config.yml` add `src/assets/js/admin.js` to `PATHS.entries`.
3. Enqueue the script conditionally in `library/enqueue-scripts.php`.
4. Run `gulp` / `npm start`