title: Invisible re-captcha fully from javascript without form submission
body: <p>I am trying to use an invisible re-captcha v2 on a form that is submitted through JS. Every example I see online shows a regular plain HTML submitted form with a specified action and method, but I am using <code>preventDefault()</code> on my form to submit it with ajax. It seems like such a simple thing but I've been searching for hours and can't find a single person online who has ever done this.</p>
  
  <p>HMTL:</p>
  
  <pre><code>&lt;form id="form-login"&gt;
    &lt;!-- ...form fields... --&gt;
  
    &lt;div
        class="g-recaptcha"
        data-sitekey="&lt;site_key&gt;"
        data-size="invisible"
      &gt;&lt;/div&gt;
    &lt;button class="uk-button uk-button-primary" type="submit"&gt;Submit&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>$('#form-login').submit(function(event) {
    event.preventDefault();
  
    console.log(grecaptcha.getResponse()); // &lt;-- always comes back empty
  });
  </code></pre>
  
  <p>I can see that the captcha is initializing because I can see the icon in the bottom right.</p>
  
  <p>I've seen <code>grecaptcha.execute()</code> but it doesn't seem to do anything for me.</p>
  
  <p>There are no errors in the console.</p>