var node = document.getElementById('btn1');
if (node.accessKeyLabel) {
  node.title += ' [' + node.accessKeyLabel + ']';
} else {
  node.title += ' [' + node.accessKey + ']';
}

node.onclick = function () {
  var p = document.createElement('p');
  p.textContent = 'Clicked!';
  node.parentNode.appendChild(p);
};