using UserListRequest with REST API End Point

Hi, I am trying to get a list of users but filter it by usernames using the <UserListRequest> node. But I get an 500 error saying that the 
<UserListRequest> node wasn't expected. 

This is the end point:
api.ashx/v2/users.xml (http://telligent.com/community/developers/w/wiki/get-users.aspx)

here is some sameple code:

HttpWebRequest request = WebRequest.Create("http://mysite.com/api.ashx/v2/users.xml") as HttpWebRequest;
          request.Method = "POST";
          request.Headers.Add("Rest-Method", "PUT");
          string requestBody = "<request><UserListRequest>testuser</UserListRequest></request>";

          byte[] bytes = Encoding.UTF8.GetBytes(requestBody);
          request.ContentLength = bytes.Length;

          Stream requestStream = request.GetRequestStream();
          requestStream.Write(bytes, 0, bytes.Length);
          requestStream.Close();

          using (HttpWebResponse webResponse = request.GetResponse() as HttpWebResponse)
          {
               using (var reader = new StreamReader(webResponse.GetResponseStream()))
               {
                    Console.WriteLine(reader.ReadToEnd());
               }
          }



What should I be posting to filter by usernames correctly? 

Thanks

Adam