or
var txtCurrency = $find(rcbCurrencyId).get_inputDomElement();txtCurrency.setAttribute('test', 'test');<
ClientSettings AllowKeyboardNavigation="True" EnableRowHoverStyle="true">
<Selecting AllowRowSelect="True" />
<ClientEvents OnRowSelected="rowSelected" OnKeyPress="KeyPress" />
</ClientSettings>

<telerik:RadScriptBlock runat="server"> <script type="text/javascript"> function VALIDATEME(sender, args) { args.set_cancel(!window.confirm('Validate?')); } </script></telerik:RadScriptBlock><telerik:RadButton ID="SaveButton" runat="server" CausesValidation="true" CommandName="Save" Text="Save" OnClientClicked="VALIDATEME" ValidationGroup="Save" /><telerik:RadComboBox ID="combo" runat="server" /><asp:RequiredFieldValidator ID="validator" runat="server" ControlToValidate="combo" ValidationGroup="Save" InitialValue="" ErrorMessage="Error!" Text="*" /><asp:ValidationSummary ID="ValidationSummary" runat="server" ValidationGroup="Save" />