Read about this app (and SMART on FHIR) at shutdownhook.com. Full source is available on github at https://github.com/seanno/shutdownhook/tree/main/smart-trials.
The links below start from the SMART sandbox and simulate launching the app from within an EHR. The app has also been tested with DSTU and R4 interfaces from both Epic and Cerner; details about running in those environments are in the blog.
Live demo site - use these links if you're just exploring. You'll have to tell your browser to accept the scary embedded cert.
Use these when developing on your own machine.