How do I create a promo code or a discount for products in an online store?

You can create the promo code in the Site Settings → Payment systems → Promo codes.

Enter a combination of letters and numbers, up to 20 characters, specify the discount value as a percentage or a fixed value (the currency of your promo code matches the currency you selected in your store), specify the quantity of promo codes (if you do not need the limitation, keep the field blank or set it to 0), set an expiration date. Save the changes.

If an expiration date is selected for a promo code, the promo code will be valid until 23:59:59 of the previous day, UTC +3 time zone (Moscow).

You can create up to 500 promo codes in one project.

Go to the page with the shopping cart, and in Content, add the "Promo code" input field. The "Promo code" field will appear in the order form of the shopping cart.

Read more about creating promo codes: https://help.tilda.cc/online-store/promo-codes