"dorp down links menu" option does't remain selected

"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 understand that code
does't understand I'm entering in the same web and returns to the default:
" Please select ... "
I am new to this stuff, any help?
form:
<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>
script:
function gotosite() {
var URL =
document.event_type_selector.url_list.options[document.event_type_selector.url_list.selectedIndex].value;
window.location.href = URL;
}