This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
The Backend module Rest client UI, in "REST request" section, will show the form to send the requests. The fields are: Method, URL resource, Headers and Body. You can select the methods GET, POST, PUT or DELETE. Furthermore you can choose two types of body request: FORM, or TEXT.