Logging in on different site with VB.NET - Help Needed

Struggling to discover what is wrong with my VB.net code

    Dim user As String
    Dim pass As String
    user = "xxx"
    pass = "xxx"
    Dim request As WebRequest = WebRequest.Create("https://api.sm**k**s.com/v3/sessions/")
    request.Method = "POST"
    Dim postData As String
    postData = "{""username"": """ + user + """,""password"": """ + pass + """}"
    Dim byteArray As Byte() = System.Text.Encoding.UTF8.GetBytes(postData)
    request.ContentType = "application/json"
    request.ContentLength = byteArray.Length
    Dim dataStream As System.IO.Stream = request.GetRequestStream()
    dataStream.Write(byteArray, 0, byteArray.Length)
    dataStream.Close()
    Dim response As WebResponse = request.GetResponse()

I get a (401) unauthorized error. Ive tried asking on various forums but haven't received any useful responses. Any advice appreciated