Form inside another form usinf master page ASP.NET C#

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C#

ASP.NET

I am trying to open form inside another form on click on a button add new to insert record, but my form is in master page.. any help in this please ..
in the below code i write a script to open it in new tab , but i need to open inside the page..

What I have tried:

<%@ Page Title="" Language="C#" MasterPageFile="~/AquaBaby.master" AutoEventWireup="true" CodeFile="Menu.aspx.cs" Inherits="Menu" %>

<asp:Content ID="Content1" ContentPlaceHolderID="Title" Runat="Server">
     
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="head" Runat="Server"> 
    <link href="css/bootstrap.min.css" rel="stylesheet" />
    <link href="css/Style.css" rel="stylesheet" />
    <script src="Jquery/jquery-3.3.1.min.js"></script>
    <script src="js/bootstrap.min.js"></script>
     <script type = "text/javascript">

         function SetTarget() {

             document.forms[0].target = "_blank";

         }

</script>
 
    
</asp:Content>

<asp:Content ID="Content3" ContentPlaceHolderID="Body" Runat="Server" EnableViewState="True" > 
      
    
    <asp:Label ID="Label1" runat="server" Text="Search:" Font-Bold="True" Font-Size="Medium"></asp:Label>

    <asp:TextBox ID="Searchtxt" runat="server" Height="24px" Width="182px" CssClass="textbox1"></asp:TextBox>
      
     <asp:Button ID="Button1" runat="server" Text="Search" CssClass="btn1" Font-Bold="True" Height="27px" Width="131px"  OnClick="Button1_Click" OnClientClick = "SetTarget();" />