This environment lets you get started with developing your application without having to send out any real checks or do direct deposits. To begin, login here, go to settings, and in the developer tab, switch to the sandbox environment. Now go to settings and grab the key/secret required to make calls to this environment. The URL and authorization header will be different than the demo cURL POST seen previously.
curl --request GET \ --url 'https://api.sandbox.checkbook.io/v3/check' \ --header "Accept: application/json" \ --header 'Authorization: YOUR_SANDBOX_API_KEY:YOUR_SANDBOX_SECRET_KEY