what should i add to this code to get the product id from another page

public partial class AddToCart : System.Web.UI.Page
{
    decimal total;
    DataTable cart = null;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["currentUId"] == null)
            Response.Redirect("~/Account/Login.aspx");

        if (Session["cart"] != null)
        {
            //Response.Write(cart.Rows[0]["p_id"].ToString());
            ///read cart from session if exist
            cart = (DataTable)Session["cart"];

            //GridView4.DataSource = cart;
            //GridView4.DataBind();

            //total = Convert.ToDecimal(Session["total"].ToString());
        }
        else
        {
            //create an empty DataTable and Add some columns to it
            cart = new DataTable();
            cart.Columns.Add("p_id", typeof(int));
            cart.Columns.Add("p_name", typeof(string));
            cart.Columns.Add("p_price", typeof(decimal));
            cart.Columns.Add("p_quantity", typeof(int));
            cart.Columns.Add("item_total", typeof(decimal));
            //total = 0;

        }
        refreshTotal();

        if (Request["DelID"] != null)
        {
            for (int i = 0; i < cart.Rows.Count; i++)
            {
                if (cart.Rows[i]["p_id"].ToString() == Request["DelID"].ToString())
                    cart.Rows.Remove(cart.Rows[i]);
            }
            Session["cart"] = cart;
            refreshTotal();
        }
        if (Request["ID"] != null)
        {
            //search item in DataTable
            bool Found = false;
            for (int i = 0; i < cart.Rows.Count; i++)
            {
                if (cart.Rows[i]["p_id"].ToString() == Request["ID"].ToString())
                    Found = true;
            }
            //add to basket if not yet added by customer
            if (Found == false)
            {
                int pid = Convert.ToInt16(Request["ID"]);
                DataTable dt = new DataTable();
                using (SqlConnection conn = new SqlConnection(SqlDataSource1.ConnectionString.ToString()))
                {
                    using (SqlCommand comm = new SqlCommand("select ProductName,UnitCost from Products where ProductID = " + pid, conn))
                    {
                        conn.Open();
                        using (SqlDataReader myReader = comm.ExecuteReader())
                        {
                            dt.Load(myReader);
                        }
                        conn.Close();
                    }
                }
                // the sqldatareader reads a single row only since products have a unique p_id
                string name = (string)dt.Rows[0]["ProductName"];
                decimal price = (decimal)dt.Rows[0]["UnitCost"];

                //                Response.Write("Product : " + name + " of price: " + price + "<br/>");
                {
                    cart.Rows.Add(new object[] { Request["ID"], name, price, (int)1, (decimal)price * 1 });
                }
            }
            Session["cart"] = cart;
            refreshTotal();

        }

        if (IsPostBack == false)
        {
            GridView4.DataSource = cart;
            GridView4.DataBind();
            refreshTotal();
        }
        refreshTotal();
    }
    protected void refreshCart()
    {
        for (int i = 0; i < GridView4.Rows.Count; i++)
        {
            TextBox Tb = (TextBox)GridView4.Rows[i].FindControl("TextBoxCount");
            cart.Rows[i]["p_quantity"] = Convert.ToInt16(Tb.Text);
            //Response.Write("quantity : " + Tb.Text + " : ");

            decimal qxp = (decimal)((Convert.ToInt32(Tb.Text) * (decimal)cart.Rows[i]["p_price"]));
            cart.Rows[i]["item_total"] = qxp;
            //Response.Write("  >>  Total" + qxp.ToString() + "<br/>");
        }

        refreshTotal();
        Response.Redirect("~/Products.aspx");
    }
    protected void refreshTotal()
    {
        total = (decimal)0;
        for (int i = 0; i < cart.Rows.Count; i++)
        {
            //Response.Write(total);
            total += (decimal)(cart.Rows[i]["item_total"]);
        }
        Label1.Text = total.ToString();
        Session["total"] = total;
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        refreshCart();
        //Response.Write(total.ToString());
        //Response.Write((string)GetMaxRequest().ToString());
    }
    protected int GetMaxRequest()
    {
        using (SqlConnection conn = new SqlConnection(SqlDataSourceRequest.ConnectionString.ToString()))
        {
            SqlCommand maximum = new SqlCommand("SELECT MAX(R_Id) as maximum FROM [Requests]", conn);
            conn.Open();
            int m = Convert.ToInt32(maximum.ExecuteScalar());
            conn.Close();
            return m + 1;
        }
    }
    protected void InsertOrder(int r_id, int p_id, int p_quantity, decimal item_total)
    {

        using (SqlConnection conn = new SqlConnection(SqlDataSourceOrder.ConnectionString.ToString()))
        {
            SqlCommand cmd = new SqlCommand("INSERT INTO [Orders](R_Id, ProductID, quantity,Items_total) VALUES (" + r_id + "," + p_id + "," + p_quantity + "," + item_total + ")", conn);
            conn.Open();
            cmd.ExecuteNonQuery();
            conn.Close();
        }

    }
    protected void InsertRequest(string UId, string sdate, decimal total)
    {
        using (SqlConnection conn = new SqlConnection(SqlDataSourceRequest.ConnectionString.ToString()))
        {
            SqlCommand cmd = new SqlCommand("INSERT INTO Requests(C_Id, R_Time,R_total) VALUES ('" + UId + "','" + sdate + "'," + total + ")", conn);
            conn.Open();
            cmd.ExecuteNonQuery();
            conn.Close();
        }
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        int r_id = GetMaxRequest();
        for (int i = 0; i < cart.Rows.Count; i++)
        {
            InsertOrder((int)r_id, Convert.ToInt16(cart.Rows[i]["p_id"]), Convert.ToInt16(cart.Rows[i]["p_quantity"]), Convert.ToDecimal(cart.Rows[i]["item_total"]));
        }
        InsertRequest(Session["currentUId"].ToString(), DateTime.Now.ToString(), (decimal)total);

        //Session["ctrl"] = pnl1;
        ClientScript.RegisterStartupScript(this.GetType(), "onclick", "<script language=javascript>window.open('Print.aspx','PrintMe','height=300px,width=300px,scrollbars=1');</script>");
        //Session["total"] = null;
        Session["cart"] = null;
        Session["r_id"] = r_id;
        Response.Write("<div class=response> Thank You For Purchasing Our Products</div>");
    }
}