I’m not sure on the exact setup to get UE4 HTML5 builds to work on itch. But, don’t be afraid to try a few different combinations, itch has a publish as draft feature so you can preview your game to see if it works before pushing it live.
First off, I would try building it as HTML5 and then zipping that up and uploading that as your ZIP/HTML5 for browser project on itch and see if that works. If that doesn’t work, a possible fall-back option is to build a shipping build for each platform (windows, max, linux etc) and upload each of those to itch.