register_meta('my_post_type', 'city', [
    'type' => 'string',
    'description' => 'Cidade',
    'single' => true,
    'show_in_rest' => true
]);

register_meta('post', 'city', [
    'object_subtype' => 'my_post_type', // Limit to a post type.
    'type'           => 'string',
    'description'    => 'Cidade',
    'single'         => true,
    'show_in_rest'   => true,
]);

register_post_type(
    'my_post_type',
    array(
        'supports' => array( 'custom-fields', ... ), // custom-fields is required
        ...
    )
);

add_action( 'rest_api_init', function () {
    register_rest_field( 'my_post_type', 'city', array(
        'get_callback' => function( $post_arr ) {
            return get_post_meta( $post_arr['id'], 'city', true );
        },
    ) );
} );