As far as I know, Itch doesn't have any system that allows you to do what you're asking.
It's designed so that any game that needs to be purchased is a downloadable game, so browser-based projects can't be restricted in that way.
You could try to implement a login system yourself or some other workaround, but you'd have to find a way to do it yourself, since Itch doesn't offer it as one of its built-in options.