View file main/templates/modals/embed_post.html

<div class="confirm--modal embedpost--modal" style="display: none !important;">
	<div class="confirm--modal--inner">
		<div class="confirm--modal--body">
			<h5>{{LANG embed}}</h5>
			<textarea cols="30" rows="3" class="form-control" id="embed_post" onclick="select();document.execCommand('Copy');"></textarea>
		</div>
		<div class="confirm--modal--footer">
			<button class="btn btn-default" data-confirm--modal-dismiss>{{LANG cancel}}</button>
			<button class="btn btn-success copy-embed-code">Copy</button>
		</div>
	</div>
</div>

<script>
$(".copy-embed-code").click(function(){
    $("#embed_post").select();
    document.execCommand('copy');
});
</script>