@EXTERNAL Bui Thanh Tuan (Amaris, MS/ETA21) Thanks for reaching out to Microsoft Q&A. Apologize for the inconvenience caused on this.
Based on the shared information we have understood that you are trying to map custom dns to your app service with the existing custom domain which is hosted by a different domain provider such as GoDaddy.
First you need to delegate the domain to Azure DNS by creating the DNS zone as explained here post that you need to create the TXT and CNAME records accordingly as mentioned here.
If you don't have a custom domain yet, you can purchase an App Service domain and map the custom DNS records to your app service by following these documentation steps
Ideally, it takes up to 48hours to verify your domain owner ship.
Feel free to reach back to me if you have any further questions on this.