JAVASCRIPT

recaptcha v3

<script src='https://www.google.com/recaptcha/api.js?render=CLAVE_SITIO_WEB'> 
</script>
<script>
    grecaptcha.ready(function() {
    grecaptcha.execute('CLAVE_SITIO_WEB', {action: 'formulario'})
    .then(function(token) {
    var recaptchaResponse = document.getElementById('recaptchaResponse');
    recaptchaResponse.value = token;
    });});
</script>

recaptcha v3 JS

<script src="https://www.google.com/recaptcha/api.js?render=your reCAPTCHA site key here"></script>
<script>
    grecaptcha.ready(function() {
    // do request for recaptcha token
    // response is promise with passed token
        grecaptcha.execute('your reCAPTCHA site key here', {action:'validate_captcha'})
                  .then(function(token) {
            // add token value to form
            document.getElementById('g-recaptcha-response').value = token;
        });
    });
</script>