function create_api_posts_meta_field() {

// register_rest_field ( \'name-of-post-type\', \'name-of-field-to-return\', array-of-callbacks-and-schema() )
register_rest_field( \'post\', \'post-meta-fields\', array(
       \'get_callback\'    => \'get_post_meta_for_api\',
       \'schema\'          => null,
    )
);

// register_rest_field ( \'name-of-post-type\', \'name-of-field-to-return\', array-of-callbacks-and-schema() )
register_rest_field( \'achievement\', \'post-meta-fields\', array(
       \'get_callback\'    => \'get_post_meta_for_api\',
       \'schema\'          => null,
    )
);function get_post_meta_for_api( $object ) {
//get the id of the post object array
$post_id = $object[\'id\'];

//return the post meta
return get_post_meta( $post_id );}