Hello Roohi Belur Raheem,
Thanks for your question.
If you deleted the domain within the last 30 days, you can restore it by repurchasing the same domain through the Azure portal. See: https://azure.github.io/AppService/2017/08/08/FAQ-App-Service-Domain-and-Custom-Domains.html
If the domain registration subscription was deleted If more than 30 days have passed since deletion, recovery may not be possible
See citation here for reference:
https://learn.microsoft.com/en-us/answers/questions/2133523/restore-a-deleted-app-service-domain
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola