[Faab]

<script>
function insertText() {  
    var formObject = document.getElementById("quick_reply");
    console.log('1 ' + formObject);
    if(formObject){
      textareaObject = formObject[0];
        if(textareaObject){
          var text = document.createTextNode('Your text'); // Тут ваш текст
          textareaObject.appendChild(text);
          // console.log('child : ' + textareaObject);
        };
    };
 };
</script>


<a href="#" onclick="insertText()">insert</a>