Hello CharlieGreen123456789,
Thank you for posting in the Apple Support Communities. From our understanding, you'd like to pair a Bluetooth heart rate monitor chest strap to your Apple Watch, and be sure your movement is appropriately tracked in the Activity app. We'd like to help.
First, you might find a third-party compatible chest strap from other reliable retailers. If you're able to find one compatible with Apple Watch, it should pair using these steps:
Use AirPods and other Bluetooth accessories with Apple Watch
Once paired, the Heart Rate and Motion sensors give you exercise credit when starting a workout in the Workout app. You can learn more about using the Workout app in these support articles:
Use the Workout app on your Apple Watch
Workout types on Apple Watch
We hope this information is useful. Kind regards.