List post categories as dropdown in metabox

I’m trying to find an example on how to add a separate meta box showing all existing categories in a dropdown menu (not to replace existing category selection) and saves the category ID into a custom field.

The custom field value will be used to identify a “primary category” for each post.

The closest thing I’ve found so far is this plugin which basically does what I’m after, but it is no longer supported.

Assistance would be highly appreciated ??