C# Custom Control To Get Internal Text As String?

I'm working on a custom control that can contain some javascript, and read this out of the page into a string field.

This is a workaround for dynamic javascript inside an updatepanel.

At the moment, I've got it working, but if I try to put a server tag inside the block:

<custom:control ID="Custom" runat="server">
<%= ControlName.ClientID %>
</custom:control>

The compiler does not like it. I know these are generated at runtime, and so might not be compatible with what I'm doing, how I can get that working?

EDIT

Error message is: Code blocks are not supported in this context

EDIT 2

The control:

[DataBindingHandler("System.Web.UI.Design.TextDataBindingHandler, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"), ControlValueProperty("Text"), DefaultProperty("Text"), ParseChildren(true, "Text"), AspNetHostingPermission(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal), AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
public class CustomControl : Control, ITextControl
{
[DefaultValue(""), Bindable(true), Localizable(true)]
public string Text
{
get
{
return (string)(ViewState["Text"] ?? string.Empty);
}
set
{
ViewState["Text"] = value;
}
}
}