Convert vb .NET webservice to C~

I have the following function to retrieve an xml string from a web service (which works fine).
Does anyone know how to do the c# equivalent - new to c# and stuck!!

Public Function apwGetCHS() As String
        Dim serviceURL As String = "https://mywebsite.com/provider/proxy/caseExport/"
        Dim paramsSB As New StringBuilder
        Dim ReturnString As String = ""

        'create the parameterised string for the URL
        paramsSB.Append("CHSUserName=myusername")
        paramsSB.Append("&CHSOrganisationID=org")
        paramsSB.Append("&CHSPassword=password")
        paramsSB.Append("&CHSCRN=crn")

        Dim params As String = paramsSB.ToString

        Dim xmldoc As Object
        Dim oxmlHttp As Object

        oxmlHttp = CreateObject("WinHttp.WinHttpRequest.5.1")
        oxmlHttp.Open("POST", serviceURL, False)
        oxmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded")
        oxmlHttp.send(params)


        'msgbox "ok"
        If oxmlHttp.status = 200 Then

            '     icmd = "alert('" & oxmlHTTP.statusText & "')"
            '     
            xmldoc = CreateObject("Microsoft.XMLDOM")
            xmldoc.loadXML(oxmlHttp.responseText)
            Try
                If FilePath(23) = "" Then
                Else
                    xmldoc.Save("testxml.xml")
                End If
            Catch ex As Exception

            End Try

        Else
            'log the error
        End If

        Return ReturnString
End Function

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Cheers,
Iain