How is a Load Generator created in AWS ECS without knowing the server or port?

I want to automate the creation of load generator containers in AWS ECS. The script you have requires the host server name or IP address, and the port:

sudo docker run -p {host port}:7100 \
-d \
-e NEOLOADWEB_URL=https\://{on premise sever}\:8080 \
-e NEOLOADWEB_TOKEN={token} \
-e ZONE={zone} \
-e LG_HOST={host name or ip} \
-e LG_PORT={host port} \
neotys/neoload-loadgenerator

Perhaps there is an environmental variable I can use for the hostname, but how does the port mapping work? If I want to bring up several instances they will all need a different (and available) host port. How does that work without manual intervention?