...
'facebook_poster' => [
    'page_id' => env('FACEBOOK_PAGE_ID'),
    'access_token' => env('FACEBOOK_ACCESS_TOKEN'),
],

use NotificationChannels\FacebookPoster\FacebookPosterChannel;
use NotificationChannels\FacebookPoster\FacebookPosterPost;

class NewsWasPublished extends Notification
{

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

    /**
     * Get the Facebook post representation of the notification.
     *
     * @param  mixed  $notifiable.
     * @return \NotificationChannels\FacebookPoster\FacebookPosterPost
     */
    public function toFacebookPoster($notifiable) {
        return new FacebookPosterPost('Laravel notifications are awesome!');
    }
}

public function toFacebookPoster($notifiable) {
    return (new FacebookPosterPost('Laravel notifications are awesome!'))
        ->withLink('https://laravel.com');
}

public function routeNotificationForFacebookPoster(): array
{
    return [
        'page_id' => 'customPageId',
        'access_token' => 'customAccessToken',
    ];
}