Passing variable in URL property

I have a www.obout.com control (splitter) that takes a URL string like this:
This URL string populates a panel within the obout.com splitter.

<RightPanel  WidthDefault="750" >
        <content Url="../controls/Display_MiniBlogDetail.aspx"/>
    </RightPanel>
I also have a hidden field,   say a value like this 1252
<asp:HiddenField  ID="hidSymbolID"  runat="server" />
QUESTION: What I want to do is get the value from the hidden field onto the end of the URL string.
I tried this : <content Url="../controls/Display_MiniBlogDetail.aspx?id1=<%= hidSymbolID.ClientID %>" />


But this didnt work,  when I read the id1 like this : Request.QueryString("id1"). It reads the <% %> as a string and does not read the value inside  (ie 1252) the hidden ...