Hide Messenger and show on clicking a button.html

<!-- hide Messenger by default -->
<script>
document.addEventListener('gistReady', function () {
    gist.chat('hide');
});
</script>

<!-- simply add 'class="gist-open-chat"' to any link on your site -->
<a href="#" class="gist-open-chat">Click me</a>

<!-- place this script tag after the Gist tracking code & jQuery -->
<script>
var GIST_CHAT_SELECTOR = '.gist-open-chat';

jQuery(document).ready(function($) {
  document.addEventListener('gistChatReady', function () {
    $(GIST_CHAT_SELECTOR).click(function(e) {
      e.preventDefault();
      gist.chat('show');
      gist.chat('openNewConversation', 'INSERT PRE-POPULATED CONTENT');
    });
  });
});
</script>