@Dr. Ramkumar R Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
After provisioning the domain, you need to connect it to your ACS resource in order to send emails using it. To do this, you will use the connection string or access key of your ACS resource to send emails through the provisioned domain.
You can find the connection strings/Access key of your ACS resource in Keys blade --> under Settings section of your ACS resource in the portal as shown below.
Hope this helps and let me know if you have any further questions on this.