try to use this code it will help you
<asp:RequiredFieldValidator ID="rfvEmployeeID" runat="server" ErrorMessage="Employee ID is required information."
ControlToValidate="txtEmployeeID" ValidationGroup="Save" SetFocusOnError="True"></asp:RequiredFieldValidator>
<br />
<asp:Button ID="btnBlockUser" runat="server" Text="Block User" CssClass="submitBtn" CausesValidation="true" OnClientClick="return validate();"
OnClick="btnBlockUser_Click" ValidationGroup="Save" />
<script type="text/javascript" language="javascript" >
function validate() {
if (Page_ClientValidate())
return confirm('Are you sure want to Block this user ?');
}
</script>