Variable from Code Behind, being used on Web Form Page (VB ASP.NET)

I have a variable that in the Page_Load event of my code behind page is being used.  When the Page_Load event is over and the page loads, I need that variable to be in a link that is currently setup.  It is not a button or asp hyperlink of any sort.  The link is acutally in a part of an area map.  I just need to be able to send through the one variable when the user clicks on that image (part of the map).  Regular ASP tags are not working and I think my syntax is wrong.

"Register.aspx?Id=<%=Id%>"

How do I get my Form page to recongize that variable from my code behind?

Thanks in advance,
Eric