Sophia here from Shopify. Welcome to the Shopify Community!
The Discounts feature built into Shopify will automatically add a code entry box to your Online Store checkout. On mobile, you'll need to tap "Show order summary" to find the code entry box. On desktop, it will look something like this:
You can use Shopify's discounts feature to create a percentage discount like 10% off your order. Even if you're using one of the third-party apps for discounts to provide advanced features for your promotions, your customer will still be using this code entry box in the checkout.
Please let me know if you have any questions about the code entry box or how to create a discount code.