[PS - 1.6.1] Add reply url in email

[ihsiut]

Hi everyone, I am beginning in prestashop and I would like to update an email send from order section -> Send configured message when you are in an order customer (cf attachment 1). 

I would like to add in this email a link which allows the customer to reply like ("domain.com/contact-us?id_customer_thread=id&token=tokenID". This kind of link at the bottom of email is already available when I reply to customer in Customer/SAV section.

In my opinion, I have to modify the file AdminOrdersController especially this part of code, but I do not what I have to add

                            if (@Mail::Send((int)$order->id_lang, 'order_merchant_comment',
                                Mail::l('New message regarding your order', (int)$order->id_lang), $varsTpl, $customer->email,
                                $customer->firstname.' '.$customer->lastname, null, null, null, null, _PS_MAIL_DIR_, true, (int)$order->id_shop)) {
                                $order->update();
                                Tools::redirectAdmin(self::$currentIndex.'&id_order='.$order->id.'&vieworder&conf=11'.'&token='.$this->token);
                            }

If anyone could help, it will be very appreciated :D