<textarea id="myTextArea" style="WIDTH: 376px; HEIGHT: 160px" rows="10" cols="44"></textarea>

<input type="button" value="插入測試" onclick="InsertContent('myTextArea','我是要插入的文字');">

function InsertContent(AreaID,Content){

   var myArea = document.getElementById(AreaID);

   //IE

   if (document.selection){

      myArea.focus();

      var mySelection =document.selection.createRange();

      mySelection.text = Content;

   }else{

   //FireFox

     var myPrefix = myArea.value.substring(0, myArea.selectionStart);

     var mySuffix = myArea.value.substring(myArea.selectionEnd);

     myArea.value = myPrefix + Content + mySuffix;

   }

}