Thank you for posting your query on Q&A.
I understand that you want to create an email address associated with your Azure Managed Domain. Microsoft Entra tenants come with an initial domain name like, domainname.onmicrosoft.com. which we can’t change or delete the initial domain name, but you can add your organization’s name to the initial domain.
To achieve your ask, create an email address for your Azure tenant domain, you need to create an Azure Communication Services Email resource and connect it to your domain. For more information, please refer How to add Azure Managed Domains to Email Communication Service
I hope this information helps! please Feel free to ask any questions you may have.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.