Thanks Guleria for your kind response, but I still get just a blank space.
Maybe the issue is somewhere else. I put the code into the liquid file.
The liquid file is inserted by the command {%- render ‘customer-box’ -%} via a custom liquid with the theme editor.
So far it is working with just HTML, but not with {{ product.metafields.custom.add_to_cart_button_fact.value }} or {{ product.metafields.custom.add_to_cart_button_fact }}

