You'll first need to create an alerts channel in Slack. You can do this by logging into your teams Slack portal and building your own Incoming Webhooks integration. You'll be provided with a Webhook URL which looks something like this: https://hooks.slack.com/services/<REMOVED>/<REMOVED>/<REMOVED>
When configuring a notification group in Iris to use Slack, you will need to provide the Webhook URL from the process above and the channel name. You can optionally provide a name for your bot otherwise it will default to "IRIS Alerts".