If you read the page carefully, it explains how to do this.
http.client.request(...) returns a request object. You can run various methods on this object before make the call out to the external web service. These include:
- Adding parameters via the parameter() method
- Adding a fragment via the fragment() method
- Adding cookies via the cookie method()
- Adding body content to the request using the body() method
When you've used these methods to add anything you need to the request, you execute the request using one of the methods: get, post, put, ...
Here is a code example (taken straight from the page). It creates a request, adds the body content, and executes the HTTP POST method:
<#assign http_client_request = http.client.request("http", "www.somewebservicedomain.com", "/some/web/service").body("{ 'application':'lithium', 'user':'johnD' }", "application/json").post() />