1. Click Settings (the gear icon), then look for Applications in the left menu. Select Register new application.

    Be sure to set the Callback URL to use the path /auth/github/callback at your site domain. e.g., http://discuss.example.com/auth/github/callback

  2. After creating your application, you should see the Client ID and Client Secret. Enter those in the Discourse fields:


That’s it!

Source: https://meta.discourse.org/t/configuring-github-login-for-discourse/13745