I presume this is in Tock? Have you tried just adding the JS to the top of a page in source using Content Editor?
Great opportunity for you to learn about Wine Direct templates. You set up FTP, connect to server, download the right file(s). Edit files, retaining a backup just in case. The files should have the same name + extension, then upload them to the server via FTP to replace. Then parse the right pages in designer launch. Put the rest of the code into the page, save, preview, make sure you’re refreshed and that it is working. Otherwise revert to original file and scratch head. You probably don’t want to distract people in the checkout areas with a visit us carrot so probably don’t need to mess with those patcular pages that work differently.