The light turns on automatically when the power of the electric power assist is turned on.
It is also possible to turn off and on the light by pressing the + button on the power controller (or by pressing and holding the UP button on the blue controller).
Reference: How to Use an Electric-Assist Bike
If the light does not turn on by pressing the + button on the power controller (or by pressing and holding the UP button on the blue controller), please report it.
Charichari App > Help > Report