To measure your heart rate, make sure your device is comfortably fitted against your wrist and not loose. The heart rate sensor on the underside of the Bio 2 Plus needs to be in direct contact with your skin.
On your device, swipe to the right of the time/date and step count screens until you see the heart icon. Make sure you are sitting or standing as still as possible while the device is measuring. The less you move, the more accurate and speedy the heart rate measurement will be.
In addition, your device is regularly measuring and capturing your heart rate. Your heart rate information can be viewed in the Striiv app by selecting the heart rate tile on the app home screen. If you wish, continuous heart rate tracking can be toggled off to save battery: