<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
if you don't want to use ValidatorCalloutExtender in RequiredFieldValidator use display=static/dynamic
<br />
<asp:Button ID="Button1" runat="server" Text="Button" ValidationGroup="vg1" onclick="Button1_Click" />
<asp:RequiredFieldValidator ID="RequiredFieldValidator1"
ControlToValidate="TextBox1"
ValidationGroup="vg1"
runat="server"
Display="None
" ErrorMessage="its
required."></asp:RequiredFieldValidator>
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender1" runat="server"
TargetControlID="RequiredFieldValidator1">
</asp:ValidatorCalloutExtender>