There's an OOB incident for this purpose called "Application Process Unavailable (unexpected)" and this is when an agent disconnects but we didn't expect it. If you edit that Incident, you can add actions to be notified however you'd like.
In addition to what Brett mentioned, you can also add a measure called 'Connected Agents' under Server Side - Functional Health...and create an incident using that measure with a Lower Threshold set to, for example 3, and with Aggregation as Count and Evaluation Timeframe as 1 min...basically when you subscribe this measure, each agent when active will show count = 1 every 10s -- so in a min you will have count = 6, if an agent disconnects the count wont be updated so within the current minute or next the count will be <= 3 and it will send an alert.
Hello @Opu Z.
Open/Edit system profile > Incidents. Here you will find OOB the incident called 'Application Process Unavailable (unexpected)'. You can configure email notifications in the action tab for the respected user/group to receive the message.
The incident purpose is that the connection to a previously connected Application Process/Agent has been lost and agent has not been able to disconnect.