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.
My phone (oneplus3) has ant+ built in, i think it's relatively common. Yours might not, or you might just need to download the ant+ software from play store.
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.