These codes are for the direct-download edition from refreshyourmac.com (not the Mac App Store edition).
1. Download RefreshYourMac (macOS 15 or later) from https://refreshyourmac.com and open it.
2. Open https://caddora.lemonsqueezy.com/checkout/buy/b72b79d3-409a-4f6e-a5e7-9cc89dd79a08 , enter your promo code in the discount code field (the total becomes $0.00), enter your email address and complete the order.
3. Your licence key is shown on the order confirmation and sent by email.
4. In RefreshYourMac, choose RefreshYourMac > Settings (Command-comma), open the Licence tab, paste the key into "Licence key" and click Activate.