On https://marked.js.org/using_pro#use it's being said I can use marked.use(myExtension, extension2, extension3); to load my custom extension but this doesn't work.

On https://marked.js.org/using_pro#extensions you learn that you actually have to use marked.use({ extensions: [description] }); with the key extensions.

The first version does not work but there's also no error message. Just nothing happens which is bad because you start guessing around why nothing happens and why your extension is not being used.