The simplest route on Drupal 9/10 is the Asset Injector contrib module.
- Install and enable the Asset Injector module.
- Go to Configuration → Development → Asset Injector → JS Injector.
- Click Add JS Injector, paste only the JavaScript portion of your Accessibird snippet, and set Code injection scope to Footer.
- Under Conditions, leave defaults so it loads on every page, then save.
Prefer not to use a contrib module? Create a Custom Block of type Full HTML, paste the snippet, and place it in a region rendered site-wide such as the footer.
