I use this one-
https://www.amazon.com/Onlyesh-Bluet...=bluetooth+hrm
Cheaper than the name brands and it sends out both a BT and ANT+ signal so I can use it for Zwift and also have it sync with my on-bike computer while riding outdoors. It's cheap but has worked perfectly other than some issues with the contacts getting corroded after washing the strap, cleaned them with a wire brush and it's back to normal.