Pairing Bluetooth Heart Rate Monitors to Apple Watch

The heart rate monitor in the Watch and - consequently - the calorie count seems very off to me vs other heart rate monitors I used.

I would like to connect the Apple Watch to my Polar FT Bluetooth chest strap which reads my heart rate (here: http://www.polar.com/en/products/accessories/Polar_WearLink_transmitter_with_Blu etooth

Has anyone successfully done this? I cannot work out how to do this.

Thanks!

I have an Apple Watch and iPhone 6+ on latest iOS