How can I make reCAPTCHA a required field?

if you want to use the native html5 popups, than here is the solution

JavaScript:

window.addEventListener('load', () => {
  const $recaptcha = document.querySelector('#g-recaptcha-response');
  if ($recaptcha) {
    $recaptcha.setAttribute('required', 'required');
  }
})

CSS:

#g-recaptcha-response {
  display: block !important;
  position: absolute;
  margin: -78px 0 0 0 !important;
  width: 302px !important;
  height: 76px !important;
  z-index: -999999;
  opacity: 0;
}