<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
                    <script type="text/javascript">
                            $('#idcat').change(function(){
                                var val = $(this).val();
                                $('#idcat2').html('');
                                $.ajax({
                                type: 'POST',
                                url: 'templates/megawebshop/html/com_virtuemart/category/show.php',
                                data: {'model': val, 'url': val},
                                success: function(data) {
                                    var obj = JSON.parse(data);
                                    obj.forEach(function(item) {
                                        $('#idcat2').append($("<option></option>").attr("value",item).text(item));
                                    })
                                }
                                });
                            });
                             $('#idcat2').change(function(){
                                var val = $(this).val();
                                $('#redirect').attr("value", $url)
                            });
                    </script>
                    <input id="redirect" type="submit" name="submit"    value="Кнопка" />

      // файл show.php

      $MODEL = $_POST['model'];
      $URL = $_POST['url'];


    if (isset($MODEL) && !empty($MODEL)) {

    $query = "SELECT model FROM filtr_db WHERE marka='$MODEL'";
    $DB = mysql_query($query);
    //  Обработаем
    $JSON = array();
     while ($row = mysql_fetch_assoc($DB)) {
     $JSON[] = $row['model'];
   }

    //  Выводим
    echo json_encode($JSON);
    } else {
      echo '[]';
      };

   if (isset($URL) && !empty($URL)) {
   $query = "SELECT url FROM filtr_db WHERE model='$URL'";
   $DB = mysql_query($query);
   $url = array();
   while ($row=mysql_fetch_assoc($DB))
            {
            $url = $row['url'];

            }
  }

$hostname = "localhost";
$username = "root";
$password = "";
$dbName = "";


mysql_connect($hostname, $username, $password) or die ("Не могу создать соединение");
mysql_query('SET NAMES utf8') or header('Location: Error');

mysql_select_db($dbName) or die (mysql_error());


 $MODEL = $_POST['model'];


if (isset($MODEL) && !empty($MODEL)) {
    $query = "SELECT model, url FROM filtr_db WHERE marka='$MODEL'";
    $DB = mysql_query($query);
    $JSON = array();
    while ($row = mysql_fetch_assoc($DB)) {
        $JSON[$row['model']] = $row['url'];
    }

    echo json_encode($JSON);
} else {
    echo '[]';
}

success: function(data) {
    var obj = JSON.parse(data);
    $.each(obj, function( index, value ) {
      $('#idcat2').append($("<option></option>").attr("value",index).text(value));
    });
}

<script type="text/javascript">
     $('#idcat2').change(function(){
        var val = $(this).val();
        window.location.href = val;
    });
</script>