How can I get heart rate info on a Vivoactive 4s

I'm trying to access heart rate info on my Vivoactive 4s, unsuccessfully so far.  I've tried

ActivityMonitor.Info.currentHeartRate,
ActivityMonitor.Info.heartRateSample
ActivityMonitor.Info.heartRate
Activity.Info.currentHeartRate

none of which give me a reading.

How can I get the heart rate from my Vivoactive 4s?