You will need to tell the customer to the use the code "Code1" in E-junkie cart for them to take advantage of the discount.
If you setup a discount code for a product, then you will need to give them the code. If you want the discount to apply automatically, that can be configured from E-junkie Seller Admin > Cart Discounts, but that will be for the whole order, not just for a specific product.
You will need to change the buttons on the site only if you are using our "Buy Now" buttons.
If you want to set a discount for "all" your products, the best is to go to E-junkie Seller Admin Cart Discounts and create 50% discount on on item total with no discount code, no minimum amount and no usage limit.