when I print this
{{ product.metafields.custom.add_product_to_show }}
I got this gid://shopify/Product/8019965837629
how can I show the above product title and description?
This is not working
{{ product.metafields.custom.add_product_to_show.title }}
You need to fix your Liquid Syntax. Metafields are a key:value store, namespaced. So in your case:
namespace is: custom (wow, clever!!)
key is: add_product_to_show
So what you really want, is the value now, and that you can get as:
{{ product.metafields.custom.add_product_to_show.value }}