Thanks for the additional info. You will need to create a CNAME record with your DNS provider. The steps are outlined here with an example of the CNAME record that you will need to use (step 3) - https://learn.microsoft.com/en-us/azure/frontdoor/standard-premium/how-to-add-custom-domain#associate-the-custom-domain-with-your-front-door-endpoint
You have another issue here however in that you are using an apex (root) domain. This is not currently supported by Azure Front Door - https://learn.microsoft.com/en-us/azure/frontdoor/standard-premium/how-to-add-custom-domain#add-a-new-custom-domain
If it helps, I wrote a blog recently outlining various ways to get around this issue - https://azurealan.ie/2021/08/20/how-to-use-root-domains-with-azure-front-door/