Documentation: Notifynder Docu
composer require astrotomic/notifynder-sender-slack
Add the following string to config/app.php
Providers array:
Astrotomic\Notifynder\NotifynderSenderSlackServiceProvider::class,
Add the following array to config/notifynder.php
'senders' => [
    'slack' => [
        'webhook' => 'https://hooks.slack.com/...',
        'store' => false, // wether you want to also store the notifications in database
    ],
],Register the sender callback in your app/Providers/AppServiceProvider.php
<?php
namespace App\Providers;
use Illuminate\Support\ServiceProvider;
use Astrotomic\Notifynder\Senders\SlackSender;
use Maknz\Slack\Message as SlackMessage;
use Fenos\Notifynder\Builder\Notification;
class AppServiceProvider extends ServiceProvider
{
    public function boot()
    {
        app('notifynder.sender')->setCallback(SlackSender::class, function (SlackMessage $message, Notification $notification) {
            // handle the message and append the from, to, icon and so on
            // https://github.com/maknz/slack#explicit-message-creation
            // you don't have to set the message text, by default (if empty) it is set in the sender itself
            // just return the message, don't send it - otherwise you will get the message two times
            return $message;
        });
    }
}