On your app create a redirection to:
https://github.com/login/oauth/authorize?client_id=YOUR_APP_CLIENT_ID&redirect_uri=YOUR_CALLBACK_URL&scope=THE_ACCESS_SCOPE&state=YOUR_STRING&allow_signup=true
The YOUR_CALLBACK_URL is the url off your app site, on this case it will be http://localhost:3333/user
It will be redirected to the YOUR_CALLBACK_URL with the query params code and state. You will use the code to make a POST request to: https://github.com/login/oauth/access_token with some params on the body: