That's not how one would use a Master Page.
The page you're trying to use is a sub-page (essentially) of the Master Page that already has the form defined: you simply need to supply everything else wrapped as follows (as an example):
<%@ Page AutoEventWireup="True" Codebehind="Default.aspx.cs" Inherits="NameSpace.Default" Language="C#" MasterPageFile="~/Site.Master" %>
<asp:Content ID="contentMain" ContentPlaceHolderID="cphMaster" runat="server">
<p>Insert page content...</p>
</asp:Content>
Quite simplistically you would create code to redirect your page from the onclick of the submit button, either in code behind (by using an asp:Button control or form javascript or by whatever means you feel is appropriate for your process.
The page you have included above is stand alone and is not designed to be used from within a Master Page.