It is not documented, but after you create your app on the eCW Sandbox, you have to reach out to eCW support so they can "activate" the app on their side. Once they activate the app, you have to add the eCW "FHIR R4 Sandbox EMR" under Customers on your sandbox configuration. Before they "activate" the app on their side, all you'll ever get when you try to "launch" the sandbox app is a 403 error that redirects you to the smart-on-fhir documentation.