I have registered a custom foo post type:

register_post_type('foo', [
    'rewrite'         => ['slug' => 'foo'],
    'capability_type' => 'post',
]);

If I have created a foo with a slug of bar I can access it with

/

I'd like to add an additional parameter to the end:

/

It looks like I need to add a ep_mask key to the rewrite array, but I am not sure how. The baz part will vary and be used to generate the page differently.