When your campaign is published, you can add promo codes to your campaign. You will be able to create promo codes for your campaign from this icon in the image
After you click on it, you will be able to generate a promo code and input the amount of discount you want to set for that promo code for that campaign. You can run that promo for maximum 7 days.
Remember, your promo amount can not be more than the commission you have set for the campagin.
For example, if you have set your commission 100 Tk for a campaign, you can only allow upto 100 Tk as discount for that campaign as your promo discount. No more than that.