On the watch:
Wake up watch by tapping the Touch Bar and scroll up for the menu. Choose "Status" and Cor will automatically start measuring. After the measurement is successful, the display will be updated in real time until the display is closed.
On the app:
Open the "Mi Fit" app and make sure the Cor is connected. Tap "Status" on the lower left of screen, choose "Heart rate", tap "Measure" at the bottom. After a few moments the app will show your current Heart Rate.