How can I add a prefix to the signature. I use the following code:

' Sign an XML file and save the signature in a new file.
Public Shared Sub SignXmlFile(FileName As String, SignedFileName As String, 
Key As RSA)
   ' Create a new XML document.
   Dim doc As New XmlDocument()

   ' Format the document to ignore white spaces.
   doc.PreserveWhitespace = False

   ' Load the passed XML file using it's name.
   doc.Load(New XmlTextReader(FileName))

   ' Create a SignedXml object.
   Dim signedXml As New SignedXml(doc)

   ' Add the key to the SignedXml document.
   signedXml.SigningKey = Key

   ' Create a reference to be signed.
   Dim reference As New Reference()
   reference.Uri = ""

   ' Add an enveloped transformation to the reference.
   Dim env As New XmlDsigEnvelopedSignatureTransform()
   reference.AddTransform(env)

   ' Add the reference to the SignedXml object.
   signedXml.AddReference(reference)


   ' Add an RSAKeyValue KeyInfo (optional; helps recipient find key to 
validate).
   Dim keyInfo As New KeyInfo()
   keyInfo.AddClause(New RSAKeyValue(CType(Key, RSA)))
   signedXml.KeyInfo = keyInfo

   ' Compute the signature.
   signedXml.ComputeSignature()

   ' Get the XML representation of the signature and save
   ' it to an XmlElement object.
   Dim xmlDigitalSignature As XmlElement = signedXml.GetXml()

   ' Append the element to the XML document.
   doc.DocumentElement.AppendChild(doc.ImportNode(xmlDigitalSignature, 
True))


   If TypeOf doc.FirstChild Is XmlDeclaration Then
      doc.RemoveChild(doc.FirstChild)
   End If

   ' Save the signed XML document to a file specified
   ' using the passed string.
   Dim xmltw As New XmlTextWriter(SignedFileName, New UTF8Encoding(False))
   doc.WriteTo(xmltw)
   xmltw.Close()
End Sub

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Thanks a lot in advance
Kurt