As I said, the fastest solution is to create a component. You can create one in few clicks with the UI: Developer’s guide to Discourse Themes You can follow that but instead of importing, you create a “new” one. Then, once component is created, you can edit it to add HTML/CSS/JS.
I don’t have the time to make screenshot, and the guide already shows you what you need… You don’t need to read everything, you can be done in less than one minute.