To do this, you have to broadcast your heart rate - and I wasn't aware until just now that the Fenix 5 series watch will actually do that. Assuming that you're connecting to Zwift with an ANT+ dongle (or using a North Pole ANT+ to BLE converter) you should, in theory, be able to do what you're wanting. Start here:
That said, be advised that though gap has closed significantly over the past 2 years or so, optical HRMs still aren't as reliable or accurate as dedicated strap devices at high heart rates (~150+). You may be better off with a strap depending on your training goals.
