@Sanjana Pushkar Thanks for reaching out. It looks like you are using the built-in service bus connector. Unfortunately, right now the built in service bus connector doesn't support sending messages to the dead letter queue. Right now, only Azure service bus connector supports the Dead-letter the message in a topic subscription action.
The workaround would be leveraging the Azure service bus connector rather than built-in. The product team is currently working on more operation in pipeline for service bus built-in connector and you can find more details on the road map here.
Let me know if you have any queries or concerns.
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