@sick anvil short answer: I have not been able to do that yet.
Long answer:
It would be incredibly inaccurate. My first prototype used the same kind of sensor (optical) as a smart watch, and results were terrible.
Outside of that, I would need to write my own smart watch app (for all platforms) to feed the data. In the end though, because of the first part of my answer, I don't think it would be worthwhile. The physical ECG (3 lead) is FAR more accurate.
If you want to DM me to discuss further, you are welcome to. But yeah, using a smart watch would be very difficult...
EDIT: You only said heart rate... not graphing? In which case... Let me dig in and see what APIs are available for Android Wear (the only watch I have uses that), but I'm skeptical about it still. On top of building the app for the watch, it would then have to communicate the heart rate to the PC somehow.. may be able to do that over direct WiFi, or even bluetooth, but then the latency worries me (jump scare, don't see a result for 4 seconds, would suck)