AsyncCallback of BackgroundWorker Webservice reference Function - vb.net

Question

I am trying to build a new class from where I can call functions with a background worker from a web service in a windows Phone 8.1 silverlight app.
calling page
LoginPage.xaml.vb
Partial Public Class LoginPage
Private sub Logon()
WebServiceHelper.a(WebServiceHelper.Functions.Logon)
LoginFinish()
End Sub
End Class
WebserviceHelper
WebServiceHelper.vb
Public Class WebServiceHelper
Public Shared Sub a(ByVal _Task As Functions)
If _Task = Functions.Logon Then
_Service.BeginLogonWindowsPhone(usr, pass, New AsyncCallback(AddressOf ResultBackGroundTask), result)
End If
End Sub
Public Shared Sub ResultBackGroundTask(ByVal result As Object)
If _Task = Functions.Logon Then
ResultObject = result
End If
End Sub
End Class
The problem is when I call webservice.a() the AsyncCallback ResultBackGroundTask doesn’t fire in time. Instead LoginFinish is called resulting in an error because the resultobject isn’t initialized yet.
I've tried:
Task.Factory.FromAsync(_Service.BeginLogonWindowsPhone, ResultBackGroundTask, usr, pass, Nothing)
But I get an error:
Argument not specified for parameter 'asyncState' of
'Public Function BeginLogonWindowsPhone(userName As String, password As String, callback As System.AsyncCallback, asyncState As Object) As System.IAsyncResult'.
The function I try to call is:
<System.ServiceModel.OperationContractAttribute(AsyncPattern:=true, Action:="http://test.com/LogonWindowsPhone", ReplyAction:="*"), _
System.ServiceModel.XmlSerializerFormatAttribute(SupportFaults:=true)> _
Function BeginLogonWindowsPhone(ByVal userName As String, ByVal password As String, ByVal callback As System.AsyncCallback, ByVal asyncState As Object) As System.IAsyncResult
This function is automatically generated in the webservice reference.
I use the Webservice to connect with the clients database, for security reasons.