add master page to an existing project in asp.net

Add MasterPage to the page directive like below

<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" Title="Untitled Page" %>
asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
Content goes here
/asp:Content>

Your Master page must contain ContentPlaceHolder like below

<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MasterPage.master.cs" Inherits="MasterPage" %>
"-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
"http://www.w3.org/1999/xhtml" >
"server">
Untitled Page
"form1" runat="server">
"ContentPlaceHolder1" runat="server">

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