OnChange not reading attribute - tinymce

Question

I have this
tinyMCE.init({
// General options
selector: 'textarea',
forced_root_block: "",
theme: "silver",
paste_text_sticky: true,
paste_text_sticky_default: true,
plugins: "paste,autolink,contextmenu,fullscreen,nonbreaking,template",
//plugins: "wordcount,paste,autolink,advlink,fullscreen,nonbreaking,template,inlinepopups,style",
// Theme options
//theme_advanced_buttons1: "bold,italic,underline,strikethrough,|,undo,redo,|,justifyleft,justifycenter,justifyfull,|,link,unlink,|,fullscreen",
//theme_advanced_buttons2: "cut,copy,paste,|,cleanup,removeformat,|,bullist,numlist,outdent,indent,hr,charmap",
//theme_advanced_buttons1: "cut,copy,paste,removeformat,|,undo,redo,|,bullist,numlist,outdent,indent,|,formatselect,bold,italic,underline,forecolor,backcolor,|,justifyleft,justifycenter,justifyfull,|,link,unlink,|,fullscreen",
theme_advanced_buttons1: "cut,copy,paste,removeformat,|,undo,redo,|,bullist,numlist,outdent,indent,|,bold,italic,underline,|,fullscreen",
//theme_advanced_blockformats: 'p,h1,h2,h4,h4,h5,h6',
theme_advanced_toolbar_location: "top",
theme_advanced_toolbar_align: "left",
//theme_advanced_statusbar_location : "bottom",
theme_advanced_resizing: true,
theme_advanced_path: false,
// Example content CSS (should be your site CSS)
selector: "textarea",
content_css: "/css/admin.css",
setup : function (ed) {
ed.on('click', function () {
new_value = tinymce.activeEditor.getContent()
alert(new_value);
new_value = new_value.replace(/,/g, "☀");
alert(tinymce.DOM.getAttrib(ed,"data-hidden"));
hidden_Field = tinymce.DOM.getAttrib(ed,"data-hidden");
$("#" + hidden_Field).val(new_value);
});
},
});
How do I get the value of the data-hidden attribute of the textarea that was filled in. tinymce.DOM.getAttrib(ed,"data-hidden"); isn't find anything as hidden_Field is, set when I do the alert. I am guessing Tinymce is ignoring it but how do I stop it from ignoring the attributeo
Thanks