getting result from a site having ajax calls Loads

im trying to parse html content loaded from an external website i have used html agility to parse the coming response

       Dim webReq = WebRequest.Create(String.Format(url))
    webReq.Method = "GET"
    Dim webResponse = webReq.GetResponse()


    Dim answer = webResponse.GetResponseStream()
    Dim _recivedAnswer = New StreamReader(answer)
    Dim res = _recivedAnswer.ReadToEnd()
    Dim htmlDocument = New HtmlAgilityPack.HtmlDocument()
    htmlDocument.LoadHtml(res)

the problem is that some of sites i have to parse uses ajax calls with timers to load up Page Contents how can i let my request read the changes of the response made by site ajax calls