How to find Category ObjectId?

I want to update my documentation from local files. I'm using npx rdme docs, as specified in this recipe: https://docs.readme.com/recipes/sync-directory-of-files-via-cli

However, it errors if I don't have the ObjectId of a pre-existing category for my project.

I created a category in the readme UI but can't find the ObjectId anywhere on the UI.

Where do I find the category ObjectId?

Alternatively, how can I create a category using the API?