Help with my javascript

Code:

<form method="post" name="my_form">
<input value="" name="URL" placeholder="Long URL"/>
<button onclick="myFunction()">Get</button>
</form>
<script>
function myFunction() {
    var myWindow = window.open("http://ouo.io/api/aX285oEC?s=+'URL'+", "", "width=400,height=400");
}
</script>

when i input the url then i click Get button, but why the url not include in the popup windows?
screenshot:


i want to the url include in popup when i click Get button, plz help