How prevent cross-site request forgery (csrf) attacks in asp.net web forms

Question

In my ASP web application, while I'm doing the pen test using OWASP, it shows

form does not contain an anti-csrf token

So I read several articles to avoid this issue. I found this solution from Stackoverflow. As he saying I need to add this code <%= System.Web.Helpers.AntiForgery.GetHtml() %> inside the web form.But My .aspx code as follows.

<%@ Page Title="" Language="C#" MasterPageFile="~/Common/OLTMaster.Master" AutoEventWireup="true"
    CodeBehind="MyAccount.aspx.cs" Inherits="LTSWeb.Admin.MyAccount" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
    <script type="text/javascript" src="../Scripts/common.js"></script>

    <asp:UpdatePanel ID="UpdatePanel1" runat="server">

        <ContentTemplate>
            <table cellpadding="0" cellspacing="0" width="100%">
                <tr>
                    <td align="left" valign="top">
                        <div class="MyAccount">
                            <div class="sectionContentWrapUp">
                                <table style="width: 100%" border="0" cellpadding="0" cellspacing="0">
                                    <tr>
                                        <td align="center" style="padding-bottom: 10px;">
                                            <table style="width: 100%;" class="gridOutLine striped-table">
                                                <tr>
                                                    <td align="right" width="50%">User Name:&nbsp;
                                                    </td>
                                                    <td align="left" width="50%">
                                                        <asp:TextBox ID="txtUserName" runat="server" CssClass="txtbox" Width="120px"
                                                            MaxLength="30" TabIndex="1"></asp:TextBox>
                                                    </td>
                                                </tr>
                                                <tr>
                                                    <td align="right" width="50%">Password:&nbsp;
                                                    </td>
                                                    <td align="left" width="50%">
                                                        <asp:RequiredFieldValidator ID="rvPassword" runat="server" ControlToValidate="txtPassword"
                                                            CssClass="errorMsg" ErrorMessage="Invalid password" ValidationGroup="vgUser">*</asp:RequiredFieldValidator>
                                                    </td>
                                                </tr>                                               
                                            </table>
                                        </td>
                                    </tr>
                                    <tr>
                                        <td align="left">
                                            <asp:ImageButton ID="btnSaveUser" runat="server" CssClass="buttonHover" OnClick="btnSaveUser_Click"
                                                SkinID="btnSave" />
                                        </td>
                                    </tr>
                                </table>
                            </div>
                        </div>
                    </td>
                </tr>
            </table>
        </ContentTemplate>
    </asp:UpdatePanel>
</asp:Content>

And this is how my aspx.cs

protected void Page_Load(object sender, EventArgs e)
{
    try
    {

        if (!Page.IsPostBack)
        {
            //Some code here
        }
    }
    catch (Exception ex)
    {
        Logger.LogErrorEvent(ex);
    }
}

protected void btnSaveUser_Click(object sender, ImageClickEventArgs e)
{
   // user data save code goes here
}

According to this answer. How I need to change my code. please help me. And I found another answer to prevent CSRF attack. But in my Master page does no contain Page_Init function. Do I need to add it manually?