Hi Andresliebrecht,
I’m not going to lie, this is a bit more complicated that just placing the code somewhere.
You’ll need have a “Partners Portal” to set up a new App and create a “Shopify Function”. There are docs and tutorials to assist: https://shopify.dev/docs/apps/build/functions. As well as lots of tutorials on youtube.
However if you don’t have access to a portal or you’re not up for the coding challenge there might be a developer that you can reach out to to assist you.