The first step in sending Sucuri monitoring alerts to Slack is to get your Slack webhook:
Choose your Slack channel.
Click Add Incoming WebHooks integration
Copy your Slack webhook.
Paste your Slack webhook into your Sucuri Slack Alert settings. Click Add Slack Account
Check your Slack channel for the Slack alerts enabled message.
That's it! You did it!
If you are still not sure what do, please feel free to open a support ticket with the Sucuri team.