Can I apply a CiviDscount discount when embedding a contribution page in a WordPress page?
CiviCRM only picks out a discrete list of shortcode parameters: see the code here.  Adding an additional shortcode parameter will not make CiviCRM recognize it and add it to the rewritten request.

Unfortunately, without rewriting CiviCRM's WordPress integration, you won't be able to get this to work via shortcode.  You'll have to use the big long URL instead.  If you're trying to generate a friendly URL, you might have to manually set a rewrite.