You login to it with your nostr identity. You'll need a "nsec bunker" which is a browser plugin. Here's one for firefox: https://addons.mozilla.org/en-US/firefox/addon/nos2x-fox/ and chrome https://chromewebstore.google.com/detail/nos2x/kpgefcfmnafjgpblomihpgmejjdanjjp?hl=en-US

You add your "nsec" (private key) to the extension, then you can click "login" at the top right at zap.stream and login with it.