If you'd like to include a promo code for the Product Hunt community, you can include one during the posting flow. You can add the text of the like "10% off", etc. You can then include the promo code itself and specific a date for the promo to expire from the post.
If you've already posted the product, you can select the downward arrow next to the title and then select 'Edit'. On the edit page, you can add the promo information.