There is an option to use service discovery, If you want to enable it you may do it while you are creating the ECS service.

The namespace name is the keyword after the dot(.), in our case it will be .local
.
There is also an option to add the Service discovery name*
this is the keyword before the dot(.).
Ref: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service-discovery.html
Update: You don't need it to point it to NLB if you are using service discovery option of ECS. There will be absolutely no role of the target group with it. ECS service will directly point a DNS name to your containers. If you want to use Load balancer bases service discovery then it's a different story altogether, Then you have to create a private hosted zone yourself and point it to your load balancer. But in the end, you can only choose one.