Obout combo box auto capitalize

Lang: VB.net application.

I've an obout combobox with default 3 values in dropdown list. I've also enabled custom text. I would like to autocapitalize every letter that is typed inside combobox. obout combobox doesn't have autocapitalize property. I tried the following. cbosuffix is the combobox id. But seems like it's going null. I would like to do this client side on key press.

Javascript

$('#cboSuffix').keyup(function() {
    this.value = this.value.toUpperCase();
});

VB

<td align="left" colspan="2" valign="top">
    <asp:Label ID="lblSuffix" runat="server" CssClass="text" Text="Suffix"></asp:Label>
</td>
<td valign="top" align="left" colspan="2">
    <asp:UpdatePanel ID="updpnlSuffix" runat="server" UpdateMode="Conditional">
        <ContentTemplate>
            <obout:ComboBox ID="cboSuffix" runat="server" FolderStyle="..Obout StyleOboutComboBox" AllowCustomText="true" EnableViewState="true" Width="100px" MenuWidth="100px" AutoValidate="true">
                <obout:ComboBoxItem ID="cboItemSuffix" runat="server" Text="None" Value="null" />
            </obout:ComboBox>
        </ContentTemplate>
    </asp:UpdatePanel>
</td>