I don't think I fully understand your question.
Do you have a listbox on one page, and want to pass it to a textbox on another page? If so, you can use the get functionality in browser by doing something like the following:
Code:
Public sub button_click(sender as object, e as eventargs)
'navigate to "http://www.mylink.com?email=" + mylistbox.Items(mylistbox.SelectedIndex).ToString
End Sub
And on the page with the textbox:
Code:
String email = Request.QueryString("email")
Me.TextBox1.Text = email
Code may not be perfect, but it should get you started : ).
Justin