How to trigger an alert when a message in a topic moved to DeadLetterQueue in ServiceBus

@Arun Kumar KS Thanks for reaching out. You can refer to this document to get started on how to analyze service bus metrics. The available message metrics are listed here.
As per your requirement you need to create the alert on Count of dead-lettered messages in a Queue/Topic metric and as per your requirement you can set the condition and evaluation.
Now you can perform what Actions you want to perform on this alert.

For more details on monitoring alerts, you can refer to this document. For creating alert, please refer to this document.

In case if you are looking to process the message when the message is received in dead letter queue then you can leverage the azure function service bus trigger or logic app workflows with service bus trigger.

Feel free to get back to me if you need any assistance.

Enable diagnostic logs for your Service Bus namespace

Log Analytics query that retrieves events related to messages being moved to the DLQ. A sample query might look like this:

AzureDiagnostics | where ResourceType == "SERVICEBUS" | where Category == "Deadletter" | where Message contains "moved to DeadLetter"

Create an Azure Monitor alert based on the Log Analytics query. You can do this by going to the Log Analytics workspace in the Azure portal, selecting "Alerts" in the left-hand menu, and creating a new alert based on the query you created in step 2. Configure the alert to send an email or other notification when triggered.

HOPE THIS HELP

I solved the alerting by creating Alert rule under the service bus namespace. You can filter the alert by specific topic. The filter by topic was what I was missing in the Alert originally. Now I create alert rule for each topic I want to monitor and set the alert to trigger anytime theres more that 1 message in Dead-Letter queue. I have deleted all the old Dead-Letter message to start from 0 count.