This works with an ESP32, I have not tested with an Arduino, but it should work. It relies on the WiFi.H Library written by some genius
#include "WiFi.h"
void setup(){
Serial.begin(115200);
WiFi.mode(WIFI_MODE_STA);
Serial.println(WiFi.macAddress());
}
void loop(){
Serial.print(" I want a Big MAC ");
Serial.println(WiFi.macAddress());
delay(800);
}