This topic has been broached a few times, but it doesn't seem to have been updated in a while.
https://community.fitbit.com/t5/Web-API-Development/Launching-fitbit-ios-app-from-my-own-ios-app/td-
Is there a url I can put into my iOS/Android app that will open the FitBit app? Or is the accepted solution still to use the Get Devices endpoint?