First, find the "Referrals" page, highlighted below, and click on it.
Once you've navigated to the Referrals page, find the "Enter Code" section pictured below. You should see a text field that says "code," and a "Submit" button. To enter your promotional code or referral code, either type or copy-paste the code here and then hit submit. Make sure you have the exact code, otherwise your submission will not be successful.
Once you've successfully submitted your promo code, you should see a progress bar appear which will track how much of your bonus that you've earned.
Remember, you can only use one promo or referral code on your account, so make sure you've got the right code or your referrer will not get their earning bonus. If you want to learn how to send referral codes, check this guide. If you want more general information about referrals, read the FAQs on the referral page, or contact Salad Support.