$.ajax({
             type: "get",
             url: "/Post/Reply/" + postId,
             success: function (data) {
                 var html= $(data);
                 $('#popup_name').append(html);
             },
             dataType: "html"
         });

public ActionResult Reply(int postId)
{
    return  PartialView("WritePostUserControl", new ReplyPostModel(postId));
}