Use Request.Form["custom"] to access your hidden field value on button click. Then use Split method to split two different variables web and pid, again use split method to split variables with their values.
HTML
<input type="hidden" id="custom" name="custom" value="web=1&pid=10000" />
<br />
web :
<asp:Label ID="label1" runat="server"></asp:Label>
<br />
pid :
<asp:Label ID="label2" runat="server"></asp:Label>
<br /><br />
<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="Submit" />
Code
protected void Submit(object sender, EventArgs e)
{
string custom = Request.Form["custom"];
this.label1.Text = custom.Split('&')[0].Split('=')[1];
this.label2.Text = custom.Split('&')[1].Split('=')[1];
}
Output
web : 1
pid : 10000