The app key and secret is necessary to configure your application with OAuth. Follow the steps below to find the on our developer portal.
Step 1 - Go the App tab on the developer portal:
Step 2 - Open your app by clicking on the card. This will open a window where you can find app details such as key and secret:
In your OAuth setup you should use the app key as "Client Key" and app secret as "Client Secret".
More Resources
GitHub - JS samples for authentication