This is Summer from the Social Care team at Shopify. Welcome to Shopify Community!
The Buy X Get Y discount code would be perfect for this setup. We have a general guide on creating this discount on this page, but I've also outlined the steps to achieve the specific Buy X Get Y setup you need below.
Your discount code is then ready. Please note your customers need to enter the code during checkout to receive the discount, and they also need to add the free item to the cart. If you would like the free item to be added to the cart automatically, an app would be needed for this setup. You can check out EasyGift: Auto Add to Cart.
To share this discount with customers, you may send the discount code via email or display it on your website. This will inform your existing customers about the code. Additionally, you can share the discount code through an ad campaign as an opportunity to draw visitors to your store. If you're not running any ad campaigns or you are but need a way to optimize them, I suggest having a look at the free social media marketing courses on our Shopify Compass page. We've got a lot of guides on advertising on different platforms, so make sure you spread this discount far and wide to attract sales.
I hope this helps! Don't hesitate to respond to this post if you have questions about the discount code or your store.