Which heart rate monitors can I connect to my eBike? 

The eBike Flow app supports the Apple Watch heart rate monitor and all heart rate monitors that use the official Heart Rate Profile 1.0, standardized according to Bluetooth®SIG. You can display your heart rate on all displays of the smart system (exception: Intuvia 100) as well as on the Ride screen of the eBike Flow app.

Check whether your heart rate monitor uses the Heart Rate Profile 1.0 by starting a pairing via the Flow app:  Settings > My eBike > eBike Pass > Components > Add new device > Other

How to connect your Apple Watch to the eBike Flow app can be found here.