Code for designer page :
<asp:TextBox ID="TextBox1" runat="server" MaxLength="11" oncopy="return false" oncut="return false" onpaste="return false"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
ControlToValidate="TextBox1" ErrorMessage="cannt be blank"
ValidationGroup="myValidator"></asp:RequiredFieldValidator>
<br />
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click"
style="height: 26px" ValidationGroup="myValidator" />
Mark my answer , if it fulfills your requirement.
Thanks,
Arshad