Currently, the only thing you can specify is a static URL:
You can't specify request type (GET, POST, PUT, etc.) or things like content type, request body, etc. It also doesn't let you embed datasets from the business process. These shortcomings make this service task nearly useless. We have several web services (both ones we have developed, and third-party ones we use) that are designed to consume data that is coming from Forms. However, as things stand, we have to use Workflow's Web Request activity, which makes our systems architecturally more complex than they need to be.