So, quick answer is that we don’t have a specific field for this type of thing. However, register_taxonomy DOES have a spot named 'meta_box_cb' that you can provide a function callback to be used to render the metabox in question. That makes you responsible for outputting everything.
meta_box_cb
(callback) (optional) Provide a callback function name for the meta box display. (Available since 3.8)
Default: null
Note: Defaults to the categories meta box (post_categories_meta_box() in meta-boxes.php) for hierarchical taxonomies and the tags meta box (post_tags_meta_box()) for non-hierarchical taxonomies. No meta box is shown if set to false.
From https://codex.www.ads-software.com/Function_Reference/register_taxonomy
So, how do you provide a value for a field that we don’t have a spot for. That’s actually pretty easy, especially if you’re familiar with filters and hooks. Please see the highlighted code at https://github.com/WebDevStudios/custom-post-type-ui/blob/release/custom-post-type-ui.php#L580-L592
With this, you have one last chance to intercept all of the arguments that will be passed into register_taxonomy, and change, remove, add, etc, before returning. You can do it for all, you can use the extra parameters to do so conditionally. Really up to you. Just be sure to pass back the array to the CPTUI code so it can complete the registration. This will allow you to add the meta_box_cb index and a value for it to call instead.
Should be all you need, but if you’re confused about anything, let me know and I will answer the best I can.
