[bes]

<style>
.div {
	position: fixed; 
	left: 20%; 
	top: 20%; 
	height: 60%; 
	width: 60%; 
	background: lightgrey;
	z-index: 10;
	display: none;
}

</style>

<button class="open">click</button>
<textarea></textarea>

<div class="div">
	<input>
	<input type="button" value="insert">
</div>


<script>
window.onload = function () {
	var but = document.body.children[0];
	var ta = document.body.children[1];
	var div = document.body.children[2];
	var inp = div.children[0];
	var ins = div.children[1];
	
	but.onclick = function () {
		var s = div.style;
		if (s.display != 'block') {
			s.display = 'block';
		} else {
			s.display = 'none';
		}
	}
	
	ins.onclick = function () {
		ta.value += '[js]' + inp.value + '[/js]';
	}

}
</script>