ProxyPanel/app/Notifications/Custom.php

<?php

namespace App\Notifications;

use App\Channels\BarkChannel;
use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Notification;

class Custom extends Notification implements ShouldQueue
{
    use Queueable;

    private $title;
    private $content;

    public function __construct($title, $content)
    {
        $this->title = $title;
        $this->content = $content;
    }

    public function via($notifiable)
    {
        return $notifiable ?? ['mail', BarkChannel::class];
    }

    public function toMail($notifiable)
    {
        return (new MailMessage)
            ->subject($this->title)
            ->markdown('mail.custom', ['content' => $this->content]);
    }

    public function toArray($notifiable)
    {
        return [
            'title'   => $this->title,
            'content' => $this->content,
        ];
    }

    public function toCustom($notifiable)
    {
        return [
            'title'   => $this->title,
            'content' => $this->content,
        ];
    }
}