No, this is not possible. With the Fitbit Versa 2, for example, there are apps that can do this. However, the measurement is quite sluggish, i.e. the watch only reacts to a sudden increase in heart rate with a delay and very moderately, and it also has to be kept firmly in place during exercise. For a more accurate measurement in real time, I still use the chest strap in training together with a sports watch. You can also find everything here at Digitec.