The Apple Watch Series 2 or later can display heart rate during workouts. The Apple Watch has a built-in wrist-based HR monitor and you can also use any external Bluetooth HR monitor compatible with your watch (like Scosche Rythm24, Polar H10, etc.). Simply pair your Bluetooth HR monitor with your watch, and the HR data will automatically be transferred to the Stryd app and your data.
If you are not getting HR on your watch, check the Privacy settings on your iPhone. Open the Settings app, and go to Privacy Settings > Health > Stryd. You can also edit these permissions from the watch itself if you are using watchOS 6.0 or later.
Here are the descriptions for each of the permissions:
Writing Permissions
Workouts - Write to Apple Health Workouts. Without this setting enabled, the other writing permissions will have no effect
Workout Routes - Write the GPS path recorded by the Stryd app to Apple Health workouts. You must also have GPS Path enabled and have Indoor mode disabled for this to work properly.
Active Energy - Write Calorie data (generated by the watch) to Apple Health workouts
Reading permission is also required
Heart Rate - Write Heart Rate data (generated by the watch) to Apple Health workouts
Reading permission is also required
Walking + Running Distance - Write distance data (generated by Stryd) to Apple Health workouts
Weight - This is not related to the Apple Watch app, but lets the weight set in the Stryd iPhone app also update your weight in Apple Health.
Reading Permissions
Active Energy - Read Calorie data from the Apple Watch. This only enables writing Calorie data to Apple Health if writing permission is also enabled.
Heart Rate - Read Heart Rate data from the Apple Watch. If you just want Heart Rate to be displayed in the app, and do not need any integration with Apple Health, this is the only permission you need to enable.
Weight - This is not related to the Apple Watch app, but lets the weight set in Apple Health also update your weight in the Stryd iPhone app. (Not recommended by Stryd)
If you want all of the above behavior, simply enable all permissions. The Stryd team, however, does not recommend enabling the permissions for weight settings.
Next, make sure the Apple watch is not using Workout Power Saving mode (which will disable HR recording). Open the Watch app on your iPhone. Select General settings, and look for Workout Power Saving mode. Make sure it is disabled.