Hi Tairi,
If I understand correctly, you are looking to set up a manual discount code. When using a manual discount code, the customer needs the correct code and the customer will apply it at checkout.
If you navigate to your discount section (discounts > create a discount > select discount type), you can select either “discount code” or “automatic discount”. Select “discount code” for it to be a manually applied discount. More information can be found on Shopify’s Discount Types.
Hopefully, that helps or is at least a starting point! Cheers!