Selector from main Load Balancer and lb_config on sub balancer


I know it is possible to automatically map new services to load balancer using selector and lb_config on a service.

But is it possible to map load balancer to load balancer using selector and lb_config. For example I would have rancher-compose inside a stack (stripped for question purpouse) like this:

    scale: 1
    start_on_create: true
       - app: stack-LB
      - path: /api
        priority: 1
        protocol: http
        service: api
        source_port: 8081
        target_port: 80

How can I make this loadbalancer auto register to main balancer? If this load balancer would be service I would just add to the port rules of lb_config:

 - target_port: 8081

but I can’t do that here because I get an error saying that source port is not present.