Depends what you site does and what it’s doing.
if you need a user to login then you need a user to login.
You shouldn’t use a “demo” account
You could create a demo flow for the user to play with but that demo would just demo your site and not login to Twitch.
Theres a lot of if/buts here because I don’t know what you are actually doing
But to answer your primary question
You can’t automatically login someone to Twitch itself
Account sharing is against the Twitch TOS
And you have no idea what someone will do being logged into some random Twitch account that is not theirs