Hi @eneskbm If you are still looking for a soltuion for this, I would like to share one of the apps available on the Shopify app store which I have used for a similar kind of functioanlity in the past. You can try using the Discount Depot APP, it allows you to set Buy X Get Y products at a discounted price of 20% all of collection Y.
Let me know if you want to get this done custom without using an app.