Adding a custom placeholder

Hello,

I need to send a mail to the author of the product when it’s sold (not the customer).
To make this I guess I need to create a custom placeholder to use in the “to” field.
I’ve read how to do it on this article on your website :
https://docs.shopmagic.app/article/174-adding-custom-placeholders
but I don’t understand anything.
Can you explain simply how it can be done?

Thank you

Regards