When you start in the Zones app on Apple Watch, it will automatically measure the heart rate.
The measured heart rate , are also registered in Health App of the iPhone.
Your workouts that were registered in the other app, it will be displayed if there are your heart rate in Health.
If your Apple Watch doesn’t stay in place, or the sensors aren’t reading your heart rate, tighten the band a bit.
For more information in here.
Heart rate is based on the heart rate registered in Apple Watch's health (HealthKit).
Therefore, the external heart rate monitor needs to be connected to Apple Watch via Bluetooth.
Please check using Bluetooth accessory with Apple Watch .
The measured heart rate is registered in iPhone's Apple Health App.
Zones' iPhone app is getting heart rate via Apple Health app.
You can see it by putting a heart rate record in Apple Health app.