To receive notifications on Slack, you first have to set up incoming webhooks on Slack as described here.
Select the workspace in which you prefer to receive notifications.
In the menu “Basic Information”, select Incoming Webhooks.
Toggle "Activate Incoming Webhooks"
Click on “Add new webhook to Workspace” and grant permission to access your Slack workspace
Note: notifications can be received either in a channel or as a direct message
Once the access is granted, you can copy the webhook URL and insert it into the Slack integration window in the workflow.
The new alert is now ready.