Use a flow in Flow Builder to configure how your Slack notification is sent using clicks, not code. But if your business needs are more complex, you can use the invocable action API to send your notification.
Flow Builder
Flow Core Action: Send Notification: After you create your custom Slack notification, your notification is available in Flow Builder as a Send Notification action. For example, if you created a custom Slack notification named My Opportunity Notification, look for the My Opportunity Notification action in the Notifications category.
Select your custom notification as an Action in the flow, then configure the recipients and record associated with your notification.
If you’re new to creating flows, learn more in Flow help.
API
sendNotification Invocable Action: Use the sendNotification invocable action to send your custom Slack notification. Each Send Notification action corresponds to a supported notification. For example, if you created a custom Slack notification named My Opportunity Notification, you can send this notification using the sendNotification/my_opportunity_notification action.