@Pavan G As per the Send Message document you need to set the BrokerProperties in the header and the value should be json object of BrokerProperties class
Header
BrokerProperties: {"ScheduledEnqueueTimeUtc":"2021-09-16T12:01:00Z"}
Content-Type : application/atom+xml;type=entry;charset=utf-8
Let me know if you are still facing the issue.
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