for example , now I have FQDN for AKS like this : xxx.southeastasia.cloudapp.azure.com
but I would like to create a subdomain something like aaa.xxx.southeastasia.cloudapp.azure.com and bbb.xxx.southeastasia.cloudapp.azure.com
I've tried to find so much but no luck
FQDN for AKS southeastasia.cloudapp.azure.com is managed directly by Azure.
Azure allows you to create only one more subdomain level of southeastasia.cloudapp.azure.com, like xxx.southeastasia.cloudapp.azure.com
You may just link your own domain to AKS FQDN, for example:
site1.example.com CNAME xxx.southeastasia.cloudapp.azure.com
site2.example.com CNAME xxx.southeastasia.cloudapp.azure.com
If you need to make an access to different services in AKS using different URLs, you should create Ingress and use routes, example:
xxx.southeastasia.cloudapp.azure.com\api
xxx.southeastasia.cloudapp.azure.com\web