How to send data to ashx-handler, but not query stirng?

I would like to open some page from external web-application in iframe.
Usualy, I would indicate URL for iframe like this: URL = http://webapp.net/handler.ashx?ID=12
BUT.
I need to send not only 'ID=12' parameter. I need to send a lot of data to handler in order to be processed.
Usualy I do it by following code:
 Dim wrq As WebRequest = WebRequest.Create(Data_WS_URL)
                wrq.Method = "POST"
             ...