Validate TextBox inside GridView FooterTemplate (FooterRow) in ASP.Net

Ref:

HTML

<asp:GridView ID="GridView1" runat="server" Width="550px" AutoGenerateColumns="false"
    AlternatingRowStyle-BackColor="#C2D69B" HeaderStyle-BackColor="green" ShowFooter="true">
    <Columns>
        <asp:TemplateField HeaderText="Customer Name">
            <ItemTemplate>
                <%# Eval("CustomerName") %>
            </ItemTemplate>
            <FooterTemplate>
                <asp:TextBox ID="txtCustomerName" runat="server" />
                <asp:RequiredFieldValidator runat="server" ControlToValidate="txtCustomerName" Display="Dynamic"
                    ErrorMessage="Required">
                </asp:RequiredFieldValidator>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Company Name">
            <ItemTemplate>
                <%# Eval("CompanyName") %>
            </ItemTemplate>
            <FooterTemplate>
                <asp:TextBox ID="txtCompanyName" runat="server" />
                <asp:RequiredFieldValidator runat="server" ControlToValidate="txtCompanyName" Display="Dynamic"
                    ErrorMessage="Required">
                </asp:RequiredFieldValidator>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="City">
            <ItemTemplate>
                <%# Eval("City") %>
            </ItemTemplate>
            <FooterTemplate>
                <asp:TextBox ID="txtCity" runat="server"></asp:TextBox>
                <asp:RequiredFieldValidator runat="server" ControlToValidate="txtCity" Display="Dynamic"
                    ErrorMessage="Required">
                </asp:RequiredFieldValidator>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:TemplateField>
            <ItemTemplate>
            </ItemTemplate>
            <FooterTemplate>
                <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="Add" CommandName="Footer" />
            </FooterTemplate>
        </asp:TemplateField>
    </Columns>
    <AlternatingRowStyle BackColor="#C2D69B" />
    <EmptyDataTemplate>
        <tr style="background-color: Green;">
            <th scope="col">
                Customer Name
            </th>
            <th scope="col">
                Company Name
            </th>
            <th scope="col">
                City
            </th>
            <th scope="col">
            </th>
        </tr>
        <tr>
            <td>
                <asp:TextBox ID="txtCustomerName" runat="server" />
                <asp:RequiredFieldValidator runat="server" ControlToValidate="txtCustomerName" Display="Dynamic"
                    ErrorMessage="Required">
                </asp:RequiredFieldValidator>
            </td>
            <td>
                <asp:TextBox ID="txtCompanyName" runat="server" />
                <asp:RequiredFieldValidator runat="server" ControlToValidate="txtCompanyName" Display="Dynamic"
                    ErrorMessage="Required">
                </asp:RequiredFieldValidator>
            </td>
            <td>
                <asp:TextBox ID="txtCity" runat="server" />
                <asp:RequiredFieldValidator runat="server" ControlToValidate="txtCity" Display="Dynamic"
                    ErrorMessage="Required">
                </asp:RequiredFieldValidator>
            </td>
            <td>
                <asp:Button ID="btnAdd" runat="server" Text="Add" OnClick="Add" CommandName="EmptyDataTemplate" />
            </td>
        </tr>
    </EmptyDataTemplate>
</asp:GridView>

Screenshot