HTTPS POST USING WEBCLIENT

I am using the following code to HTTPS post an xml file to third party server. Now I believe I am able to send the file correctly but I need to pass a HTML input field value of type hidden as well with it. If I use a simple HTML interface as shown below I am successful:

<form enctype="multipart/form-data" action="https://myurl" method="POST">
  <input type="hidden" name="MyFieldName" value="myvalue">
  <input type="file" name="Myfile">
  <br/>
  <input type="submit">
</form>

I want to simulate the exact thing in my .NET code. I am confused how to pass value of the hidden field “MyFieldName” along with the file as I am using the WebClient.UploadFile() function. I tried passing it as query string but it does not work.

Can you please suggest something, I am now desperate.

Following is the code i am using at the moment in .NET:

Private Sub testHTTPPostXmlDoc()

        Try
           
            Dim web As New System.Net.WebClient()

            web.Headers.Add("Content-Type", "multipart/form-data")
           
            ' Simulating the line <input type="hidden" name="MyFieldName" value="myvalue">
web.QueryString.Add("MyFieldName", "myvalue")
           
Dim res As Byte() = web.UploadFile("https://myurl", "POST", "C:\MyFile.xml")
            Dim response As String = System.Text.Encoding.ASCII.GetString(res)

        Catch webEx As WebException

            MsgBox(webEx.Message, MsgBoxStyle.Information, "Debug Msg")
            MsgBox(webEx.StackTrace, MsgBoxStyle.Information, "Debug Msg")

        Catch ex As Exception

            MsgBox(ex.Message, MsgBoxStyle.Information, "Debug Msg")
            MsgBox(ex.StackTrace, MsgBoxStyle.Information, "Debug Msg")

        End Try

End Sub

I BELIEVE THE ISSUE IS BECAUSE OF THE SSL CERTIFICATE. AS I AM POSTING TO HTTPS RATHER THAN HTTP, I THINK I NEED TO ENCRYPT THE DATA SOMEHOW.

CAN YOU PLEASE ASSIST. AS WHEN I DO POST FROM HTML PAGE IT WORKS FINE BUT I BELIEVE THE WEB BROWSER ENCRYPTS THE DATA REQUESTING THE CERTIFICATE PUBLIC KEY. HOW CAN I IMPLEMENT THE SAME IN MY CODE?