Espressif Systems ESP8266 Beginner's Manual page 28

If you check the page source (CTRL+U), you can see the simple HTML form that's used:
<form action="/LED" method="POST">
<input type="submit" value="Toggle LED">
</form>
Sending data to the ESP using HTTP POST
In the previous example, we sent an empty POST request to the ESP8266. In the previous chapter however, I explained that it's
possible to send all kinds of data in the body of the POST request.
In this example, I'll show you how to send a username and a password to the ESP. The ESP will then check if they are correct, and
respond to the request with the appropriate page.
#include <ESP8266WiFi.h>
#include <WiFiClient.h>
#include <ESP8266WiFiMulti.h>
#include <ESP8266mDNS.h>
#include <ESP8266WebServer.h>
ESP8266WiFiMulti wifiMulti; // Create an instance of the ESP8266WiFiMulti class, called 'wifiMulti'
ESP8266WebServer server(80); // Create a webserver object that listens for HTTP request on port 80
void handleRoot(); // function prototypes for HTTP handlers
void handleLogin();
void handleNotFound();
void setup(void){
Serial.begin(115200); // Start the Serial communication to send messages to the computer
delay(10);
Serial.println('\n');
wifiMulti.addAP("ssid_from_AP_1", "your_password_for_AP_1"); // add Wi-Fi networks you want to connect to
wifiMulti.addAP("ssid_from_AP_2", "your_password_for_AP_2");
wifiMulti.addAP("ssid_from_AP_3", "your_password_for_AP_3");
Serial.println("Connecting ...");
int i = 0;
while (wifiMulti.run() != WL_CONNECTED) { // Wait for the Wi-Fi to connect: scan for Wi-Fi networks, and connect to the strongest of
the networks above
delay(250);
Serial.print('.');
}
Serial.println('\n');
Serial.print("Connected to ");
Serial.println(WiFi.SSID()); // Tell us what network we're connected to
Serial.print("IP address:\t");