If you want to offer a discounted options to users, you would need to setup two different pages with two different event registration forms. The first page includes the discount price and the second page includes the full price. Instead of providing a discount code to the participants, you would provide them with a link to the discounted price page.

Further, if you want to restrict access to the discounted page, you may setup targeting and security on the part and page to limit it to specific users who are logged in.