组通道中用户的添加通知

我正在尝试发送一个通知,其中提到一个普通频道中的用户。这就是我所拥有的:

<?php

namespace App\Notifications;

use Illuminate\Bus\Queueable;
use Illuminate\Notifications\Messages\SlackMessage;
use Illuminate\Notifications\Notification;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;

class WeeklyTasksResponsible extends Notification
{
    use Queueable;

    protected $employee;

    /**
     * Create a new notification instance.
     *
     * @return void
     */
    public function __construct(\App\Employee $employee)
    {
        $this->employee = $employee;
    }

    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function via($notifiable)
    {
        return ['slack'];
    }


    /**
     * Get the Slack representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return SlackMessage
     */
    public function toSlack($notifiable)
    {
        return (new SlackMessage)
            ->content('Reponsible for this week is: ' . $this->employee->slack_name);
    }
}

这将在我们公司的总频道每周发送一次通知。信息是“本周负责人是:nameofuser”。问题是用户看不到这方面的通知。

我也试过这样做:

public function toSlack($notifiable)
{
    return (new SlackMessage)
        ->content('Reponsible for this week is: @' . $this->employee->slack_name);
}

我该怎么做?