Coupon codes can be created within your site id's settings under CRM menu CRM> Sites. Select the site you want to add coupon to and click on Edit under Actions. Click on Coupons, then + New to create the coupon and save. Enable the coupon(s) by selecting "Yes" under "Enable Coupons."
Note: Coupons do not apply to Shipping Charges or Ship Amounts within a product setup.