The footnote syntax is based on both the shortcode and comments formats that already exist, so no new syntax was added.
Each footnote needs a key to associate it with the corresponding % comment. These are free form, but auto-generated if you create the footnote and comment pair from the Insert menu, so no need to type any of it yourself. It's just a single click.
The % comment syntax is also used for adding a synopsis to chapter and scenes, and will be extended to other features as well.