Add extra fields to the post’s featured image

In your case, you need to add custom fields to the metabox. The full reference code of the filter is here: ‘admin_post_thumbnail_html’ – https://developer.wordpress.org/reference/hooks/admin_post_thumbnail_html/.

Let’s add an image title:

<?php
add_filter('admin_post_thumbnail_html', 'custom_featured_image_field', 10, 2);
function custom_featured_image_field($content, $post_id) {
    $custom_name    = ‘custom_featured_image_title’;
    $custom_value = esc_attr(get_post_meta($post_id, $custom_name, true));

    $output = sprintf(
        ‘<br><input type=“text” id="%1$s" name="%1$s" value="%2$s">',
        $custom_name, $custom_value
    );

    return $content .= $output;
}?>

Let’s save the newly added custom field

Here we will sanitize user data and save or update the custom featured image title field.

<?phpadd_action('save_post', 'save_custom_featured_image_field', 10, 3);
function save_custom_featured_image_field($post_ID, $post, $update) {
    $custom_name="custom_featured_image_title";
    $custom_value = isset($_REQUEST[ $custom_name ]) ? 1 : 0;
    update_post_meta($post_ID, $custom_name, $custom_value);
}?>