recaptcha-verifier.js

// set-up an invisible recaptcha. 'sendCode` is button element id
// <button id="sendCode">Send Code</button>
window.recaptchaVerifier = new firebase.auth.RecaptchaVerifier('sendCode', {
  'size': 'invisible',
  'callback': function (recapchaToken) {
    // reCAPTCHA solved, send recapchaToken and phone number to backend.
    
    // a REST call to your backend
    request.post({
      url: 'https://your-rest-api',
      body: {
        phoneNumber: document.getElementById('phoneNumber').value,
        recapchaToken,
      }
    });
  }
});

// render the rapchaVerifier. 
window.recaptchaVerifier.render().then(function (widgetId) {
  window.recaptchaWidgetId = widgetId;
});

Where do I need to put this code?