Oh, sorry, I misread that. This worked for me:
#include "Particle.h"
#include "deviceid_hal.h"
SerialLogHandler logHandler;
void setup() {
waitFor(Serial.isConnected, 10000);
delay(2000);
char buf[32];
hal_get_device_secret(buf, sizeof(buf), NULL);
Log.info("secret %s", buf);
}
void loop() {
}