Convert cURL to VB.NET

Not sure what happened there - I thought I'd submitted this but its disappeared - so sorry if my question is duplicated!

Anyway, I need to convert the following cURL command to vb.net to obtain an access token from an OAuth2 authorisation server. I'm normally reasonably competent handling HTTP requests in vb.net but I just can't get this to work. can anyone help please?

curl  -u  ID_Key:Secret_Key  https://targetURL.com  -d  'grant_type=client_credentials'

My coding is:

        Dim credstring As String = "ID_Key:Secret_Key"
        Dim authstring As String = Convert.ToBase64String(Encoding.UTF8.GetBytes(credstring))

        Dim webrequest As WebRequest = webrequest.Create("https://targetURL.com?grant_type=client_credentials")

        With webrequest
            .Headers.Add("Authorization", "BASIC " & authstring)

            Dim webresponse As HttpWebResponse = .GetResponse() '

            Dim stream As System.IO.Stream = webresponse.GetResponseStream()
            Dim reader As New StreamReader(stream, Encoding.UTF8)
            Dim contents As String = reader.ReadToEnd()
            Debug.Print(contents)
        End With


Obviously the ID, Key and URL will be the real thing.

Many thanks.