passing value from one page to another using PreviousPage.FindControl property in ASP.NET 2.0

Hi,
I am trying to transfer txtOppId.Text value from Default.aspx page to EnterBid.aspx page using PreviousPage.FindControl()  method but unable to do so.  Here's my code for your review.
Deafult.aspx
txtOppId.Text = Request.QueryString["oppId"];
...........................
...................... 
 
EnterBid.aspx:

protected void Page_Load(object sender, EventArgs e)
{
 
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