Alert based on field

This is my scenario. I have one index named "employee" which have 3 fields name, salary and created_at. I want to trigger an alert everytime someone index the document with salary below certain value. For example if salary < 20000 I want to trigger an alert. I don't understand which type of rules should I use.
I am not sure if there is any way to alert based on field values.

Index threshold rule is what you want:

Thanks for the answer. My conditions is working, but alert is not triggered when I add another document in the index. What should I select in "Notify me"?
I only want to notify when someone index the document which meet the condition

Are you sure the new document added has a created_at date that falls within the condition set in the alert logic (i.e. in the last 5 days)?