Logout from logged in captive portal https://iqfareez.com/blog/hacking-wifi-captive-portal-1

captive_portal_logout.ino

#include <WiFi.h>
#include <Wire.h>
#include <HTTPClient.h>

#define AP_SSID "IIUM-Student"
#define AP_PASSWORD "" // leaves empty if open wifi

const String username = "111111";
const String password = "blahblah";

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);

  // Connecting to the WIFI AP
  WiFi.begin(AP_SSID, AP_PASSWORD);

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

  Serial.println("Connected to the WiFi network");

  // Logout from Captive Portal
  HTTPClient logout;
  logout.begin("https://captiveportalmahallahgombak.iium.edu.my/cgi-bin/login?cmd=logout");
  int res = logout.GET();

  Serial.println(res);

  if (res == 200) {
    Serial.println("logged out successfully");
  }

  logout.end();
}

void loop() {
  // put your main code here, to run repeatedly:

}