TVM.
So to enable standby you should use;
sensor_t* sensor = esp_camera_sensor_get();
sensor->set_reg(sensor, 0x3008, 0x40, true ? 0x40 : 0x00);
And to disable it;
sensor_t* sensor = esp_camera_sensor_get();
sensor->set_reg(sensor, 0x3008, 0x40, false ? 0x40 : 0x00);