How to bind a Camunda ExternalTaskClient to a local port?

I am working with Camunda’s ExternalTaskClient Java client. I am able to subscribe to a topic successfully, but this uses a random port on the host machine to communicate with Camunda.

How can I tell the ExternalTaskClient to use a custom outgoing port? Here is my client so far:

ExternalTaskClient client = ExternalTaskClient.create().baseUrl("http://1.2.3.4:8080/engine-rest").build();

client.subscribe("my-topic").handler(new MyHandler()).open();

Thanks!