Assign post category automatically based on author

Hi,

I would like to automatically assign a category to a post based on the author’s ID.

I have tried to start from this thread:
https://ww.wp.xz.cn/support/topic/automatically-add-posts-to-a-category-conditionally?replies=4
but that code alone does not work.

I have also looked at hacking this plugin that works (https://ww.wp.xz.cn/plugins/author-category/) with which an author can be assigned a category manually in backend, but I have to admit this is beyond my current skills.

Has anyone any idea how to achieve this?