I am using Recurly's JavaScript API to process subscriptions payments.
I want to implement Google's reCaptcha V3 API to the Recurly's self-hosted page.

<script src="https://js.recurly.com/v4/recurly.js"></script>

recurly.configure({
    publicKey : 'xxx-xxx',
    required  : ['cvv', 'address1', 'city', 'state', 'country', 'postal_code'], 
});

// When a customer hits their 'enter' key while in a field
recurly.on('field:submit', function (event) {
    $('form').submit();        
});

// On form submit, we stop submission to go get the token
$('form').on('submit', function (event) {
    // Prevent the form from submitting while we retrieve the token from Recurly
    event.preventDefault();
    // Reset the errors display
    $('#errors').text('');
    $('input').removeClass('error');
    // Disable the submit button
    $('button').prop('disabled', true);
    var form = this;
    // Now we call recurly.token with the form. It goes to Recurly servers
    // to tokenize the credit card information, then injects the token into the
    // data-recurly="token" field above
    recurly.token(form, function (err, token) {
        // send any errors to the error function below
        if (err) error(err);
        // Otherwise we continue with the form submission
        else form.submit();
    });
});

Things is, Google's API implementation is something like this :

<input type="hidden" name="recaptcha_response" id="recaptchaResponse">
<button type="submit" id="btn-submit" class="g-recaptcha" data-sitekey="xxxxxxxxx" data-callback='onSubmit' data-action='submit'>Submit</button>

<script>
function onSubmit(token)
{
    document.getElementById("recaptchaResponse").value = token;
    document.getElementById("frm-subscribe").submit();
}
</script>

Both have their own version of onSubmit. How do I include Google's one into Recurly's ?