The authentication system for the FeedPress API is token-based.
In order to authenticate an user, you have to send the user to the /login.json method with a callback parameter:
GET https://api.feedpress.com/login.json?key=APIKey&callback=http://mywebsite.com/authThe callback parameter must be an URL of your website. When the user is successfully authenticated, they will be redirected to the callback URL with a token parameter added.
You can therefore use this token to make API requests on behalf of the user.