When you request a feature flag, you need to pass in a user object. The only mandatory field is the key. In JSON, this looks like this:
The only requirement for keys is that they uniquely identify the user visiting your site. For authenticated users, an e-mail address or the primary key of that user from your database makes a perfect user key. For unauthenticated users, a session ID or UUID works– as long as the same session ID or UUID is associated with the same anonymous user on repeat visits to your site.