400 bad request when querying microsoft graph with http post - vb.net

Question

I've been doing some testing with Microsoft Graph and I seem to have hit a brick wall; Wondering if anyone can give me a steer in the right direction.
The following code is from my test app (vb)...
Imports System.Net
Imports System.Text
Imports System.IO
Imports Newtonsoft.Json.Linq
Class MainWindow
Public Shared graph_url As String = "https://graph.microsoft.com/v1.0/"
Public Shared br As String = ControlChars.NewLine
Dim myscope As String = "https://graph.microsoft.com/.default"
Dim mysecret As String = "zzx...BX-"
Dim mytenantid As String = "7aa6d...d409199"
Dim myclientid As String = "4e37...5a59"
Dim myuri As String = "https://login.microsoftonline.com/" & mytenantid & "/oauth2/v2.0/token"
Dim mytoken As String = ""
Public Function HTTP_Post(ByVal url As String, ByVal postdata As String)
Try
Dim encoding As New UTF8Encoding
Dim postReq As HttpWebRequest = DirectCast(WebRequest.Create(url), HttpWebRequest)
postReq.Headers.Add("Authorization", "Bearer " & mytoken)
postReq.Method = "POST"
postReq.PreAuthenticate = True
Dim postreqstream As Stream = postReq.GetRequestStream()
If Not postdata = Nothing Then
Dim byteData As Byte() = encoding.GetBytes(postdata)
postreqstream.Write(byteData, 0, byteData.Length)
End If
postreqstream.Close()
request_header.Text = postReq.Headers.ToString
Dim postresponse As HttpWebResponse = DirectCast(postReq.GetResponse(), HttpWebResponse)
Dim postreqreader As New StreamReader(postresponse.GetResponseStream())
Dim response As String = postreqreader.ReadToEnd
Return (response)
Catch ex As Exception
user_results.Text = ex.Message.ToString
End Try
End Function
Public Function GetNewToken()
Console.WriteLine(myuri)
Dim post_data As String = "client_id=" & myclientid & "&client_secret=" & mysecret & "&scope=" & myscope & "&grant_type=client_credentials"
Dim token As String = HTTP_Post(myuri, post_data)
get_result.Text = JObject.Parse(token).SelectToken("access_token")
mytoken = JObject.Parse(token).SelectToken("access_token")
End Function
Private Sub Button_Click(sender As Object, e As RoutedEventArgs)
GetNewToken()
End Sub
Private Sub usrget_Copy_Click(sender As Object, e As RoutedEventArgs) Handles usrget_Copy.Click
Dim url As String = "https://graph.microsoft.com/v1.0/users/*{myUPN}*"
Dim post_data As String = Nothing
Dim return_data As String = HTTP_Post(url, post_data)
req_url.Text = url
End Sub
End Class
The GetNewToken() function works fine and I retrive a valid token without an issue.
When i try and use that token to query a user, i get 400 Bad request - no additional info as such; just that.
I've examined the headers of my POST request and tried it several different ways, but cannot seem to overcome this; As far as i can tell my request is formatted as specified in the documentation for Microsoft Graph.
Have also googled the hell out of this to see if there was something obvious or simple I have overlooked; I've seen a few posts on this topic where people suggested setting the Authorization header before anything else - I tried that and it made no difference.
A few people also suggected setting the Accept to 'application/json'; I also tried that and it made no difference.
I've outputted some of the details to the gui and grabbed a screenshot so you can see what I'm seeing...
Interestingly, If i try the 'Query users' before i grab a token, I get a 401 unauthorized which would suggest that the request itself is working correctly.
As i mentioned, this is a testing app whilst I get some functionality working (some data obscured for obvious reasons); I'm not concerned about the tidyness of code or anything like that at this point.
If anyone is able to help it would be very much appriciated.
Thanks in advance.