Add RankMath meta field to REST API

add-rankmath-metafield-to-api.php

<!--  This exposes the following meta field to posts in REST API: rank_math_focus_keyword, rank_math_title, rank_math_description -->

function register_rank_math_custom_fields_rest_api()
{
    $meta_keys = [
        'rank_math_focus_keyword',
        'rank_math_title',
        'rank_math_description'
    ];

    foreach ($meta_keys as $meta_key) {
        register_meta('post', $meta_key, [
            'type'         => 'string',
            'description'  => 'Campo Rank Math personalizzato',
            'single'       => true,
            'show_in_rest' => true,
            'auth_callback' => function () {
                return current_user_can('edit_posts');
            }
        ]);
    }
}
add_action('init', 'register_rank_math_custom_fields_rest_api');