Hello, Greetings! of the day. As mentioned in project description, your requirement is to add discount code, applicable to order on checkout page, on your shopify website. In shopify admin panel, the discount code can be added as follows:
1) In shopify admin panel, there is "Discounts" page. On "Discounts" page, when "Create Discount" button is clicked, it will display options to select discount type.
2) Regarding order discount, the discount type "Amount off order, Order discount" should be selected. When discount type is selected, it will display settings page for discount code. On this page, the settings for discount code can be done.
If you get any issues in implementing discount code functionality on your shopify website, you may tell. I have much experience in working on projects, using shopify. Your work will be done, according to requirements.
Thanks.