If you want to use the Epicollect5 API to get data for a private project, you will have to register your "app" to get authentication details to exchange with the Epicollect5 server.
Go to your Apps page under the Developers section:
Click on the "Create New App" button
Give a name to your App and click on "Create"
Notice you need to register your app only once for a project and reuse the Client ID and Client Secret on your applications.