Adding product Y automatically when clicking add to cart for product X

[Stormmmneo]

Hi!
May I know or get some help regarding adding a freebie product Y that shows up in the cart drawer when clicking on add-to-cart for product X? 

An example is the image below where the second product is added automatically when a customer add the crystal hair to cart.

I've tried a few ways with the product ID through the const form but i think i may be adding it wrongly



thanks so much!