Start

curl "http://localhost:7999/1/config/set?emulate_motion=1

Stop

curl "http://localhost:7999/1/config/set?emulate_motion=0

Normally port 7999 / motion_control_port is set to only listen on localhost.
The configuration file is located at

/etc/motioneye/motioneye.conf

#the TCP port that motion HTTP control interface listens on
motion_control_port 7999

# whether motion HTTP control interface listens on
# localhost or on all interfaces
motion_control_localhost true