Sinric Pro Contact Sensor. Tutorial: https://help.sinric.pro/pages/tutorials/contact-sensors/contact.html

contact-sensor.ino

#if defined(ESP8266)
  #define SENSOR_PIN        D2
#elif defined(ESP32) 
  #define SENSOR_PIN        26
#elif (ARDUINO_ARCH_RP2040)
  #define SENSOR_PIN        5
#endif

int newState;
int oldState;

void setup() {
  Serial.begin(9600);
  pinMode(SENSOR_PIN, INPUT_PULLUP);

  oldState = digitalRead(SENSOR_PIN); // read the starting state
}

void loop() {
  newState = digitalRead(SENSOR_PIN); // read current state

  if(oldState != newState) {
    if (newState == HIGH) {
      Serial.println("Sliding window is open");
    } else {
      Serial.println("Sliding window is closed");
    }
    oldState = newState;
  }  
}