protoype field

prototype.js

function addFieldForm(collectionHolder, $newLinkLi)
{
	var prototype = collectionHolder.attr('data-prototype');

	var newForm = prototype.replace(/__name__/g, collectionHolder.children().length);

	var $newFormLi = jQuery('<li></li>').append(newForm);
	$newLinkLi.before($newFormLi);
}

var collectionHolder = jQuery('#unimog_contenttype_fieldbundle_contenttypetype_fields');

var $addFieldLink = jQuery('<a href="#" class="add_field_link">Add a field</a>');
var $newLinkLi = jQuery('<li></li>').append($addFieldLink);

jQuery(document).ready(function()
{
	collectionHolder.append($newLinkLi);

	$addFieldLink.on('click', function(e){
		e.preventDefault();

		addFieldForm(collectionHolder, $newLinkLi)
	});
});