# .env file
SLACK_WEBHOOK_BASE_URL='https://hooks.slack.com'
APP_SLACK_WEBHOOK_YOUR_CHANNEL_ADDRESS='your channel address'

We define the required settings in the env file.
// config/packages/framework.yaml
framework:
    ...
    http_client:
        scoped_clients:
            slack_webhook_client:
                base_uri: '%env(resolve:SLACK_WEBHOOK_BASE_URL)%'
                timeout: 2.0*/

We define scoped HTTP clients in our framework configuration file. I couldn’t find any Attribute usage for this in the documentation.
<?php
namespace App\Service;

use Psr\Log\LoggerInterface;
use Symfony\Component\DependencyInjection\Attribute\Autowire;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Contracts\HttpClient\HttpClientInterface;
use Throwable;

readonly class SlackService
{
    public function __construct(
        #[Autowire(service: 'slack_webhook_client')]
        private HttpClientInterface $slackClient,
        #[Autowire(env: 'APP_SLACK_WEBHOOK_YOUR_CHANNEL_ADDRESS')]
        private string              $yourChannelAddress,
        private LoggerInterface     $logger
    )
    {
    }

    public function sendText(string $text): void
    {
        try {
            $this->slackClient->request(
                Request::METHOD_POST,
                $this->yourChannelAddress,
                [
                    'json' => ['text' => $text],
                ]
            );
        } catch (Throwable $exception) {
            $this->logger->warning($exception->getMessage());
        }
    }
}