If you’re just going direct to the API endpoint in the browser, then in most cases without a browser extension to allow you to edit request headers you wont be able to do that with the new Helix API. For simple testing, a lot of people like Postman https://www.getpostman.com/ it’s a simple tool that’s free and lets you set headers and all that you’ll need.
As for using fetch if you check out the docs for it https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch and scroll to “Supplying request options” you’ll see that when making a fetch request, as well as a URL you can also provide an options object, one of the options you can set is the headers, in this case Client-ID