The documentation needs to be changed with the latest update to Cloud Manager. See: https://docs.cloud.mongodb.com/tutorial/add-monitored-deployment-to-automation/.

This is the updated process:

• On the deployment page, click on the button with the wrench and a green plus icon for the cluster
• Click the checkbox, then click Continue
• All the servers already have automation agents so click Initialize Automation
• Wait for Cloud Manager to finish it's process
• Click on Review Deployment
• Click on Confirm & Deploy