Sends a POST request to the server.
POST is used to send data to the remote HTTP endpoint/script.
Provide the data you need to send via the Content parameter. If Content is omitted, and only Url is set, the data will be streamed dynamically using the DynamicDataNeeded event.
To post a web form's data (including file attachments to the form) use PostWebForm method.
Note that any parameters you intend to pass to this method should be properly encoded before this method is called.