How to measure heart rate using an oscilloscope?

Generally it doesn't work. You have to filter out all the unwanted signals and amplify the small signals buried in the noise. Probably the largest signal is from the side of the thorax to the center of the chest. But the signal is present from the left hand/wrist to the right hand/wrist. It is also usually done with a differential amplifier and a scope front end is normally single ended.