A VERY quickly made plugin that adds an 'Add New' link after any Meta Box Select Advanced field that uses select2 for selecting related posts.

Trying to satisfy my needs described here: https://metabox.io/support/topic/is-it-possible-to-inline-add-a-new-record-on-a-related-post-type/

• For use with Meta Box – https://metabox.io/

add-add-new-to-meta-box-select-advanced.php adds Thickbox & custom JS to the Post Edit & New screens. add_add_new_to_mb.js places an "Add New" button (after a setTimeout) immediately following any Select Advanced field, for "post" types only (doesn't work for adding new Users or Terms, etc).

If a user does not find the related post in the Select Advanced field:

• User can click the "Add New" button
• A modal (or new window) will open to the "Add New" screen for the related post type
• User can add a new post, then close the modal/window
• User can then search for the post they just added and it will display in the Select Advanced field

You can customize a few options in the add_add_new_to_mb.js file. Look for the aanConfig object (that stands for 'add Add New config').

You will definitely want to customize the baseUrl to match your WP site. If your site is just in the root of your domain, remove the /app from the beginning of the baseUrl.

Here are the other aanConfig options:

• useModal: boolean - determines if the "Add New" screen opens in a modal or in a new window, if set to true it a modal will open, if false a new window will open (Default: true)
• modalParams: string - currently this is pretty much irrelevant, but I were to improve the script to load an actual modal view of the Add New screen, then this would be a required parameter (Default: '&TB_iframe=true')
• modalWidth: string or number - the width of the modal (Default: '600')
• modalHeight: string or number - the height of the modal (Default: '550')
• baseUrl: string - the relative path to the Add New screen for you WP site (Default: '/app/wp-admin/post-new.php?post_type=')
• linkText: string - the text that will display inside the anchor element (Default: 'Add New')
• linkClass: string - any classes you want to add to the anchor element, remove button if you would rather just show a plain text link Note: thickbox class is automatically added if useModal is true (Default: 'button')
• linkStyle: string - any inline styles you want to apply to the anchor element (Default: 'margin-left:1rem;')

These are for my memory and easy access later:

• https://codex.wordpress.org/Javascript_Reference/ThickBox
• https://developer.wordpress.org/reference/functions/iframe_header/
• https://developer.wordpress.org/reference/functions/iframe_footer/
• https://metabox.io/support/topic/is-it-possible-to-inline-add-a-new-record-on-a-related-post-type/
• https://www.exratione.com/2018/02/the-easiest-javascript-modal-for-administrative-pages-in-wordpress-4/
• http://rizqy.me/create-modal-box-on-wordpress-dashboard/

My clients needed the ability to add a new post if it didn't already exist in the Meta Box Select Advanced dropdown. So this hacky solution opens the "Add New Post" page for whatever custom post type the select field is using. Then the user can save (publish) the new entry, close the window that opened up & do a search for what they just added and the ajax query should pick it up.

This is an EXTREMELY quick and simple solution. It has lots of potential for error, but it works great for my uses.

Contributors: Brandon Pfeiffer - @brandonjp - brandonjp.com Donate something: https://www.paypal.me/brandonjp