inline code ASP Classic style

I am no expert at this, but I need to fill a simple hidden input with a request variable.  Before you tell me about all the better ways to do this: I know... but I can't do it that way in this case.
Here is the line of code that *should* work, yet doesn't.
<asp:HiddenField runat="server" id="userAgent" value="<%# Request.UserAgent %> " /> This always has a blank value, when I am expecting data. 
Please don't tell me to do this in code-behind, I can't in this case.  I just want to fix the hidden field so that the value from the request gets populated.