Are you trying to link it to a phone? If you are, you'll need an ANT+ adaptor on your phone as the Garmin HRMs don't do Bluetooth. Mine don't, anyway. I know Strava can work with Bluetooth HRMs on a phone, I assume it'll also talk ANT+ with a dongle.
Anyone know if you can pair a garmin hrm with strava directly? Mine works on my garmin but just says searching on Strava