Can you check if there are any errors in the javascript console? Do you at least see your HTML? Is there a URL online where we can see the issue?
As long as you only want to insert something above or below Flarum single page app, creating a Flarum extension shouldn't be needed. However if you want to insert the HTML inside of a Flarum page or handle page navigation you will need to use the Flarum javascript API.