Here is a quick way to get the IP address and/or MAC address from your Arduino device as a String.

String get_mac_addr() {
    uint8_t mac[6];
    WiFi.macAddress(mac); // Put the addr in mac

    char buf[18] = "";
    snprintf(buf, sizeof(buf), "%02X:%02X:%02X:%02X:%02X:%02X", mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);

    return (String)buf;
}

String get_ip_addr() {
    IPAddress ip = WiFi.localIP();

    char buf[16];
    snprintf(buf, sizeof(buf), "%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3]);

    return (String)buf;
}

Quicky helper function to connect to WiFi on Arduino so I don't have to re-invent the wheel every time I start a new project.

void init_wifi(const char *ssid, const char *password) {
    WiFi.mode(WIFI_STA); WiFi.begin(ssid, password);

    while (WiFi.status() != WL_CONNECTED) {
        delay(500); Serial.print(".");
    }

    Serial.printf("\r\nConnected to: \"%s\"\r\n",ssid);
    Serial.print("IP address  : "); Serial.println(WiFi.localIP());
    Serial.print("MAC address : "); Serial.println(WiFi.macAddress().c_str());
}