I am just getting started with FHIR. I set up sandbox environments for AthenaHealth, EPIC, and eClinicalWorks. For AthenaHealth I am using a clientID/Secret method to get an APIKEY, and for EPIC I am using a JWT/JWK method. That latter works on EPIC, but the same method does not work for eClinicalWorks. I have had a few troubleshooting sessions with their tech support, but we have thus far not been able to figure out what the issue is.
I posted a thread on StackOverflow that gives an summary / overview of my issue.
Stack Overflow Thread
I may need to update that. I have tried a variety of troubleshooting measures, but nothing has worked thus far. I should say that I think you need to have a US or North America IP address to access their sandbox also. I might update that to also create an EC JWK at some point.