add_action( 'init', 'create_custom_tax', 0);

function create_custom_tax() {
	$labels = array(…);

	$args = array(
		'hierarchical'      => true,
		'labels'            => $labels,
		'show_ui'           => true,
		'show_admin_column' => true,
		'query_var'         => true,
		'rewrite'           => array( 'slug' => 'custom_tax' ),
		'show_in_rest'       => true,
  		'rest_base'          => 'custom-taxonomy',
  		'rest_controller_class' => 'WP_REST_Terms_Controller'
	);

	register_taxonomy( 'custom_tax', array( 'post' ), $args );
}