upload JSONobject to web server

I am writing a client application for a networked device. The device requires that control values are changed using html "POST" and sending form data containing a JSON object. The devices has some sample commands using CURL that work fine and I am able to capture the "POSTs" using Wireshark. Here is a capture of a good transaction:


I am using VB.NET to write my client application and using WebRequest I can send the value pair but the form data is missing the "json=" part of the data:


This is the code I have so far:

        Using sendto As New Net.WebClient
            Dim param As New Specialized.NameValueCollection
            param.Add(theName, theValue)
            Dim response_bytes = sendto.UploadValues(uri, "POST", param)
            Dim response_body = (New Text.UTF8Encoding).GetString(response_bytes)
        End Using

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The server rejects the post and closes the connection.

I have searched but cannot find a way to get this data correct. Any pointers would be much appreciated,
Sid.