Thank you for getting back.
Based on your statement above
I added www.csonline.com in the Azure Frontdoor custom DNS section and added Azure-generated DNS TXT record and CNAME record to public DNS.
From your question above, I understand that you are using classic Front Door SKU. For classic FrontDoor the custom domain is verified using CNAME record as documented here https://learn.microsoft.com/en-us/azure/frontdoor/front-door-custom-domain#map-the-permanent-custom-domain
For Standard & Premium SKU the custom domain is verified using DNS TXT record as documented here:
https://learn.microsoft.com/en-us/azure/frontdoor/domain#domain-validation
Can you please confirm which AFD SKU you are using?
Can you please let us know what type of CNAME record you have added to your domain in your Domain registrar's website?
For example, if your domain is registered as csonline.com on your domain registrar. Then you will add a CNAME record with the host value of "www" pointing towards <hostname>.azurefd.net. This experience may vary as per the domain registrar used. You can refer to this documentation for additional details.
Please let me know if you have any additional questions. Thank you!