apiInstance.auth returning login page

[Jejugo]

I would like to get the code from the back end without having to open login screen and authenticating visually on the screen. Whenever I perform this:

  apiInstance.auth(responseType, clientId, redirectUri, (error, data, response) => {
    if (error) {
      console.error(error);
    } else {
      console.log('API called successfully.');
    }
  }); 

the API is called successfully but the response object is an HTML. There is no way I can retrieve this accessToken without using the browser to login and redirect to my redirect_uri, is there?