<script src="https://www.google.com/recaptcha/api.js" async defer></script>
<script>
    function onSubmit = function(token) {
    document.getElementById("admin-login").submit();
    }
</script>

<button data-sitekey="My Site Key" data-callback='onSubmit' class="btn btn-default submit g-recaptcha" name="login">Log in</button>

<script src="https://www.google.com/recaptcha/api.js" async defer></script>
<script>
    var onSubmit = function(token) {
    document.getElementById("admin-login").submit();
    }
</script>