jquery ajax调用后台方法

$.ajax({
            type: "post",
            url: "Detail.aspx/GetSignature",
            data: "{'str':'" + st + "'}",
            contentType: "application/json;charset=utf-8;",
            dataType: "json",
            success: function (data) {
               alert(data.d);
             
            },
            error: function (e) {

            }
        })

 [WebMethod]
    public static string GetSignature(string str) {
       return "test";
    }