Laravel Slack on-demand notification; where to set the webhook?

As per the Laravel docs I can do an on-demand notification in a controller like this:

use Notification;
use App\Notifications\TradeSuccessful;

$trada_data = array( 'title' => 'test', 'amount' => 123.45 )

Notification::route('slack', '#test')->notify(new TradeSuccessful($trade_data));

And in TradeSuccessful (example code):

public function toSlack($notifiable)
    {
        return (new SlackMessage)
            ->success()
            ->content('One of your invoices has been paid!')
            ->attachment(function ($attachment) use ($trade_data) {
                $attachment->title('Invoice 1322')
                    ->fields([
                    'Title' => $trade_data['title],
                    'Amount' => $trade_data['amount]
                ]);
            });
    }

Main question: when I use Notifications like this (on demand), where do I set the Slack webhook? Because in the documentation they use:

public function routeNotificationForSlack($notification)
    {
        return 'https://hooks.slack.com/services/...';
    }

But that function is defined on a Model, and when using on demand notifications nothing is defined on a Model.

via Chebli Mohamed