In order to use the MindMeister API endpoints, first you have to register an API client application.
Add an OAuth 2.0 application here. Set your application name and add one or more valid redirect URIs.
You will receive a unique client ID, which identifies your application and a client secret. This is needed when you have to authenticate your app against MindMeister's authorization server.
Hide your client secret
Important: Keep your client secret hidden and don't check it in on your version control system like Git.