.Net RESTful Web Service: sending xml file

I'm trying to send an xml file containing a list of files to a restful webservices. It 'a project in vb.net. The function that passes the data to the web services is the following:

 Private Function SendActivityToWEBSERVICE(ByVal xmlFile As String) As Boolean
    Try
        sUri = "http://localhost:35299/LiveUpdateWS/SincronizzaAttivita?d=" + xmlFile.ToString()

        Dim req As HttpWebRequest = WebRequest.Create(sUri.ToString())
        req.Method = "GET"  
        req.KeepAlive = False

        Dim response As HttpWebResponse = req.GetResponse()
        Try
            Dim xmlDoc As New Xml.XmlDocument
            xmlDoc.Load(response.GetResponseStream())

        Catch exc As XmlException
            Console.WriteLine("Eccezione " + exc.Message)
            Return False
        End Try

    Catch ex As Exception
        Console.WriteLine("Eccezione " + ex.Message)
        Return False
    End Try
    Return True
End Function

The interface of web services is as follows:

<OperationContract>
<WebGet(UriTemplate:="SincronizzaAttivita?d={sXMLFile}")>
Function SaveDataPost(sXMLFile As String) As Boolean

If the xml file that I am sending is of small size everything works fine. If I try to send large files I get the error 404.15. I saw that to send strings or data of a certain size to a web services is recommended to use POST instead of GET. To me it is unclear how to modify the above code to do it, always that this is the solution to my problem. Can you tell me what and how to change the code?