Not obvious, you put the naddr of the stream in the URL like this example. The chat doesn't render nicely in the browser, as it's built to be transparent and used in OBS. Might do some changes later to make it look better. It loads the last 10 chats and 10 zaps and then show new as they come in.

https://chat.nostria.app/naddr1qvzqqqrkvupzpn6956apxcad0mfp8grcuugdysg44eepex68h50t73zcathmfs49qqjrvefjxvurywpk94jrvcmp956rqvfc94snjdpj943xvc338yexzwpsx3snqm92y0r

A little more info available on the main page: https://chat.nostria.app/

Will add plenty more features later on.