MultiLine textbox with FilteredTextBoxExtender and new line charaters

Dear all

 I have a problem where I'm using a multi-line textbox and have an ajaxtoolkit:FilteredTextBoxExtender attached to this.

The problem is how to you set new line (when you press Enter key) as a valid character.

 The following is what I have so far.

<asp:TextBox ID="tb_Identifier"   TextMode="MultiLine" Width="400px" Height="200px"  runat="server"></asp:TextBox>

 <ajaxToolkit:FilteredTextBoxExtender ID="filter_IdentifierFilter" runat="server" TargetControlID="tb_LoginCodes" FilterType="UppercaseLetters, Numbers"/>