create the instance of the service and using that call the meethod here..
public partial class Default : System.Web.UI.Page
{
Service1 srvc = new Service1();
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
Response.Write(srvc.GetVaryByCustomString("","").ToString());
}
}