FYI for anyone else trying to do this. I was able to get the snippet that Josue posted working, but instead of the using the field name (i.e. referrer), I had to specify the index of the field as it appears in the Theme Settings > Newsletter > List fields.

		'5' => array(
			"id"=>'5', 
			"type"=>'hidden', 
			"value" => "http://google.com"
		)