<input id="btn" type="button" value="Вставить из буфера">
<div id='editableDiv' contenteditable='true'>...</div>
<script>
	btn.addEventListener('click', btnClick);
	btn.addEventListener('paste', doPaste);
	editableDiv.addEventListener('paste', doPaste);
	function btnClick(event) {
		var event = document.createEvent("Event");
		event.initEvent("paste", true, true);
		btn.dispatchEvent(event);
	}
	function doPaste(event){
		var clipboardData = event.clipboardData || window.clipboardData;
		console.log('Clipboard data: ' + clipboardData.getData('Text'));
	}
</script>