Press the button of your BPM+ once to turn it on. Note that if you press the button during a measurement, it will stop the measurement.
Press and hold the button of your BPM+ for three seconds to turn it off.
https://support.withings.com/hc/en-us/sections/6215920350481-BPM
https://support.withings.com/hc/en-us/sections/115002567047-Tutorials