How to filter GridView Based on Dynamically Added Textboxes on first row position using C# and VB.Net in ASP.Net

<asp:GridView ID="gv_siswa" runat="server" AutoGenerateColumns="False" CellPadding="4" ForeColor="#333333" GridLines="None" Font-Size="Small" EmptyDataText="No Record Display" CssClass="table">
    <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
     <Columns>
       <asp:BoundField DataField="nik" HeaderText="Nik" />
       <asp:TemplateField SortExpression="nik">
               <HeaderTemplate>
                   <asp:TextBox ID="txtnik" runat="server" placeholder="Filter Nik" AutoPostBack="true" Width="100px" OnTextChanged="txtnik_TextChanged"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>     
       <asp:BoundField DataField="nama" HeaderText="Nama" />
           <asp:TemplateField>
               <HeaderTemplate>
                   <asp:TextBox ID="txtnama" runat="server" placeholder="Filter Nama" AutoPostBack="true" Width="100px"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>                       
       <asp:BoundField DataField="fakultas" HeaderText="Fakultas" />  
           <asp:TemplateField>
               <HeaderTemplate>
                   <asp:TextBox ID="txtfakultas" runat="server" placeholder="Filter Fakultas" AutoPostBack="true" Width="100px"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>                        
       <asp:BoundField DataField="jurusan" HeaderText="Jurusan" />
           <asp:TemplateField>
               <HeaderTemplate>
                   <asp:TextBox ID="txtjurusan" runat="server" placeholder="Filter Jurusan" AutoPostBack="true" Width="100px"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>   
       <asp:BoundField DataField="nilai1" HeaderText="Nilai1" />
           <asp:TemplateField>
               <HeaderTemplate>
                   <asp:TextBox ID="txtnilai1" runat="server" placeholder="Filter Nilai1" AutoPostBack="true" Width="100px"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>   
       <asp:BoundField DataField="nilai2" HeaderText="Nilai2" />
           <asp:TemplateField>
               <HeaderTemplate>
                   <asp:TextBox ID="txtnilai2" runat="server" placeholder="Filter Nilai2" AutoPostBack="true" Width="100px"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>   
       <asp:BoundField DataField="nilai3" HeaderText="Nilai3" /> 
           <asp:TemplateField>
               <HeaderTemplate>
                   <asp:TextBox ID="txtnilai3" runat="server" placeholder="Filter Nilai3" AutoPostBack="true" Width="100px"></asp:TextBox>
               </HeaderTemplate>
           </asp:TemplateField>