RaceChrono supports the HRM profile on the Bluetooth LE, but unfortunately most Smart Watches do not implement it. The sensors that implement it are usually wrist bands, chest belts etc.

If you think that your Smart Watch is different and might implement it, you could always try to add it from RaceChrono > Settings > + Add other device > Heart rate sensor (BLE). If it does not work, it probably means your Smart Watch does not implement the HRM profile on Bluetooth LE.