@Vimalraj S Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
Due to the recent service updates, Sender usernames & user engagement tracking cannot be enabled for Azure Managed Domains or custom domains with default sending limits which is called in the documentation here as well.
For more information, see Service limits for Azure Communication Services.
If you want to add multiple sender email address & enable user engagement tracking, then you need to raise a support ticket to upgrade your service from default limits. Once you raise a support ticket our backend team will review and update them accordingly.
Hope this helps, let me know if you have any further questions on this.