“dorp down links menu” option does't remain selected

I’m doing a “dorp down links menu” for WordPress however when I click and go into “dogs” this option does’t remain selected. I know that code does’t understand I’m entering in the same web and this returns to default: ” Please select … “

any help?

<form name="event_type_selector" method="post" action="#">
	<select name="url_list" class="event-type-selector-dropdown" onchange="gotosite()">
		<option value="">Please select...</option>
		<option value="?product_cat=para-perros">dogs</option>
		<option value="?post=107&action=edit&message=6">alegoría</option>
		<option value="?post_type=product&s=muneco">Menu</option>
	</select>
</form>
<script language="javascript">
	function gotosite() {
		var URL = document.event_type_selector.url_list.options[document.event_type_selector.url_list.selectedIndex].value; window.location.href = URL;
	}
</script>