session variables and request.querytring in .net

Usually I write in Classic ASP, but now I'm stuck with a .NET code i need to change:

<%@ Page Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeBehind="Index.aspx.cs" Inherits="Pliskin.Index" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
    <style type="text/css">
    .ltor
    {
          direction:rtl;
          overflow:hidden;
          resize: none;
    }
    .ltor:focus
    {
          outline: none;
    }
    </style>
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <asp:Panel BackColor="#FFFFCC" runat="server" Width="410">
    <asp:TextBox BorderStyle="None" runat="server" ReadOnly="True" ID="Directions" TextMode="MultiLine" CssClass="ltor" Height="400" Width="400" Font-Size="X-Large" BackColor="#FFFFCC"></asp:TextBox>
    <br />
    <asp:TextBox ID="Username" runat="server"></asp:TextBox>
    <asp:Button runat="server" OnClick="Start" Text="Start" />
    </asp:Panel>
</asp:Content>

Now this code works - but I need it to include two additional things
1) To create a session variable -
<%session("id")=request.querystring("uid")%>

2) To put that value as the value of the "username" variable -
<asp:TextBox ID="Username" runat="server" value="<%=request.querystring("uid")%>" disable></asp:TextBox>

When I try to change it - the file don't work anymore