How access to HTML element page1.aspx from page2.aspx.cs

At runtime, form1 does not exist while form2 is being displayed. HTTP works over a request/response setup. The browser makes a request, the server returns the requested page. The connection dies at that point and the server forgets you even exist.

If you requested Form2, Form1 never comes into the request/response process at all. It doesn't exist. Therefore, if you need to pass a value from Form1 to Form2, you have to save that value somewhere after a form on Form1 is submitted to the server. The server can either save that value is a Session object or in a database or some other storage for use in a later request. It's also possible for the servers response to the form submission to respond with Form2 and use the value from the Form1 submission.

Have a look :

Form1 :
=======

<form id="form1" runat="server">
  <div>
      <asp:Label ID="lblForm1" runat="server" Text="Form 1 Data"></asp:Label>&nbsp;&nbsp;
      <asp:TextBox ID="txtForm1" runat="server"></asp:TextBox>&nbsp;&nbsp;
      <asp:Button ID="btnForm1" runat="server" Text="Button" onclick="btnForm1_Click" />

  </div>
  </form>

protected void btnForm1_Click(object sender, EventArgs e)
   {
       Session["data1"] = txtForm1.Text;
       Response.Redirect("Form2.aspx");
   }

Form2 :
=========

<form id="form2" runat="server">
<div>

    <asp:Label ID="lblForm2" runat="server" Text="Form 2 Data (from session variable)"></asp:Label>&nbsp;&nbsp;
    <asp:TextBox ID="txtForm2" runat="server"></asp:TextBox>&nbsp;&nbsp;

</div>
</form>

protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["data1"] != null)
        {
            txtForm2.Text = Session["data1"].ToString();
        }
    }