Slack notifications allow you to send notifications of deployments directly into a Slack room. For more information about Slack, visit their website.
To set up a new notification, click the Notifications link on the left hand side of your project, then the New Notification button to get started.
Enter your account name (your Slack domain) and the token (the last part of the webhook url) you've just generated, any customisation to the message format using variables found via the these variables link on the page, then finally the (default) option to display the message as rich text.
You can also choose when to send the notification - when the deployment starts, if it fails, or if it succeeds.
Click Create Notification once you've finished entering your configuration.