What does Ember-simple-auth check againest?

I have been looking for answer of implementing ember-simple-auth (oauth2-password-grant) for days without luck. I use firebase to sign up users, which is successful. However, when in log in page, the action of this.get('session').authenticate('authenticator:oauth2', credentials) seems causing a json error (SyntaxError: Unexpected token < in JSON at position 0) . So my first question is, in theory, how this authentication checks if use email/password is correct? meaning in which file is the "answer" located. Am i supposed to define token? if yes, i already tried "serverTokenEndpoint: 'http://localhost:4200/' or serverTokenEndpoint: 'http://localhost:4200/token" and none works. thanks.