How do I create an automatic alert sent only when a "Reply" field is filled in? "Alert Management" does not answer my question. Can i use the calculated field ? If yes, how ?
You cannot create alert for other users using "Alerts" action of SharePoint.
Alternatively, you can use designer workflow. Everytime item is update/Added the workflow will run to check if "Reply" is filled or empty. If it's filled you can send email using "Send Email" action in Workflow