Problem creating a login user via RESTful API in VB.NET

Hello all, I have the below code with which I am trying to create a login user via the RESTful API in VB.NET. The API returns that this is an invalid JSON but when I validate the JSON via any online validation site it comes back as valid. Does anyone have an idea as far as why it may be failing?

' Creates the JSON web request and returns the results
Private Function SendRequest(uri As Uri, jsonDataBytes As Byte(),contentType As String, method As String) As String
' Create initial request
Dim req As WebRequest = WebRequest.Create(uri)
req.ContentType = contentType
req.Method = method
req.ContentLength = jsonDataBytes.Length
' Establish request stream
Dim stream = req.GetRequestStream()
stream.Write(jsonDataBytes, 0, jsonDataBytes.Length)
stream.Close()
' Retrieve response
Dim response = req.GetResponse().GetResponseStream()
' Parse response
Dim reader As New StreamReader(response)
Dim res = reader.ReadToEnd()
reader.Close()
response.Close()
' Return JSON result
Return res
End Function
Public Function CCC_Test_API_AddUser() As String
CCC_Test_API_AddUser = Nothing
Dim jsonString As String
jsonString = "{""operation"":{""Details"":{""USERNAME"":""ThisIsATest"",""FIRSTNAME"":""Test"",""LASTNAME"":""User"",""EMAIL"":""TUser@test.com"",
""PHONEFACTOR_USERNAME"":""ThisIsATest""}}}"
' Work around to skirt SSL Certificate Validation Checking
System.Net.ServicePointManager.ServerCertificateValidationCallback = _
Function(se As Object, _
cert As System.Security.Cryptography.X509Certificates.X509Certificate, _
chain As System.Security.Cryptography.X509Certificates.X509Chain, _
sslerror As System.Net.Security.SslPolicyErrors) True
Dim siteUri As New Uri("https://server:7272/restapi/json/v1/user?AUTHTOKEN=F6C0CD37-5BB9-44BE-8331-FA4BC64C6071&INPUT_DATA=")
Dim data = Encoding.UTF8.GetBytes(jsonString)
Dim result_post = SendRequest(siteUri, data, "text/json", "POST")
Return result_post
End Function