<asp:TextBox ID="TextBox2" runat="server" EnableViewState="False" Width="350px"
                    Text='<%# Bind("ProductFullName")%>'></asp:TextBox>

int prodId = Convert.ToInt32(Request.QueryString["ProductId"]);
        SampleDataEntities db = new SampleDataEntities();

        var prod1 = from prod in db.Product
                    where prod.ProductId == prodId
                    select prod;

                            
        TextBox2.Text = как сюда вывести зачение из поля ProductFullName??

SampleDataEntities.Products