Ticket #23240: edit-form-test.php

<?
add_action( 'edit_form_after_title', 'test_cpt_extra_fields' );
add_action( 'edit_form_after_editor', 'test_cpt_extra_fields' );
function test_cpt_extra_fields() {
        if ( 'test-cpt' !== get_current_screen()->post_type )
                return;

?>
<h3>Something</h3>
<input type="text" class="widefat" />
<?php
}

add_action( 'init', 'register_test_cpt', 1 );
function register_test_cpt() {
        register_post_type( 'test-cpt', array(
                'label' => __('Test CPT'),
                'public' => true,
                'show_in_nav_menus' => false,
                // comment items out for testing
                'supports' => array(
                        //'title',
                        'editor',
                        'thumbnail',
                        'comments'
                ),
        ));
}