Untitled

<html>
<head>
    <script>
         function formSubmit(form) {
            window.location.href = '/invoices/' + form.userinput.value;
            return false;
         }
    </script>
    <title>test</title>
</head>

<body>
    <form name="goto" action="test.html" onSubmit="return formSubmit(this);">
        <input name="userinput" type="text">
        <input type="submit" value="try me">
    </form>
</body>