With Fargate, each task has an ENI attached to it, and that ENI uses an auto-assigned IP from the specified range of the subnet. So the short answer is, the IP can be found on the task instance page in the AWS Console. Click on the service name (default
in your case), then the Tasks tab, and then click on the task GUID-- not the Task Definition-- and you'll see your IP listed in the Network area.
Note that this IP will change every time you stop and start a new task. The most straightforward way to combat this is by using an application load balancer (ALB).