With Push Notification services, they'll ask for a file like push-worker.js or manifest.json to be placed in the root folder.

To do this you need to go to your landing page or site

Click Pages and click +Add Page

Select a static page from the dropdown.

Edit your new page, and in the HTML tab you can put your static site content.

Even if this isn't a HTML page, so in this instance, if it's a JS file, you still put it in the HTML section.

It will look something like this: