With BODY BIKE indoor cycling App version 1.6.3 or older, it is not possible to connect to your Apple Watch or any other SmartWatch as a live heart rate monitor.
This is due to the different Bluetooth protocols. The BODY BIKE indoor cycling App understands two protocols – cycling power 0x1818 and heart rate 0x180D.
Cycling power 0x1818 is used between SMART+ and the BODY BIKE indoor cycling App.
Heart rate 0x180D is used between the heart rate monitor and the BODY BIKE indoor cycling App.
As long as the watch does not transmit with the right protocol (0x180D) it cannot connect with the BODY BIKE indoor cycling App.