src\Eccube\Service\MailService.php

function sendOrderMail

$webHook        = $this->container->getParameter('sale_notify_slack_webhook');
        $channelName    = $this->container->getParameter('sale_notify_slack_channel');
        $message = [
            "username"   => "通知",
            "icon_emoji" => ":shopping_trolley:",
            "channel" => $channelName,
            "text" => $_SERVER['HTTP_HOST']."\n".
            "注文がありました。\n".
            "注文番号：".$Order->getOrderNo()."\n".
            "　会社名：".$Order->getCompanyName()."\n".
            "購入者名：".$Order->getName01().$Order->getName02()."(".$Order->getKana01().$Order->getKana02().")\n".
            "支払方法：".$Order->getPaymentMethod()."\n".
            "支払金額：￥".number_format($Order->getPaymentTotal()),
        ];
        $ch = curl_init();
        $options = [
            CURLOPT_URL => $webHook,
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_SSL_VERIFYPEER => false,
            CURLOPT_POST => true,
            CURLOPT_POSTFIELDS => http_build_query([
                'payload' => json_encode($message)
            ])
        ];
        curl_setopt_array($ch, $options);
        curl_exec($ch);
        curl_close($ch);