Click listener to ajax loaded content. Pure javascript
<p>I have this javascript which loaded some content block by ajax</p>

<pre><code>var sendContainer = document.getElementById("loadForm");

        sendContainer.addEventListener("click", function (event) {

      var xmlhttp;
            if (window.XMLHttpRequest) {
                xmlhttp = new XMLHttpRequest();
            }
            xmlhttp.onreadystatechange = function () {
                if (xmlhttp.readyState == 4 &amp;&amp; xmlhttp.status == 200) {

        // Show loaded content block in result div
                    document.getElementById("result").innerHTML = xmlhttp.responseText;

                }
            };
            var form = document.getElementById('ajax_send_message');
            var formData = new FormData(form);

            xmlhttp.open("GET", '/ajax', true);
            xmlhttp.setRequestHeader("X-Requested-With", "XMLHttpRequest");
            xmlhttp.send(formData);
 });
</code></pre>

<p>And loaded content block</p>

<pre><code>&lt;button type="button" id="ajax_submit""&gt;Send&lt;/button&gt;
</code></pre>

<p>How to force this addEventListener to get id (ajax_submit) from dynamically loaded block</p>

<pre><code>document.getElementById("ajax_submit").addEventListener("click", function (event) {
alert('WORK!');

});
</code></pre>