Tell SearchWP's indexer to index WooCommerce Order Numbers as extra metadata

functions.php

<?php

/**
 * Tell SearchWP's indexer to index WooCommerce Order Numbers as extra metadata
 * 
 * @param $extra_meta
 * @param $post_being_indexed
 *
 * @return mixed
 */
function my_searchwp_index_wc_order_number( $extra_meta, $post_being_indexed ) {

	if ( 'shop_order' !== get_post_type( $post_being_indexed ) ) {
		return $extra_meta;
	}

	// Index plain order number and these variants:
	//      order-{order_number}
	//      o-{order_number}
	$extra_meta['searchwp_order_number'] = $post_being_indexed->ID . ' order-' . $post_being_indexed->ID . ' o-' . $post_being_indexed->ID;

	return $extra_meta;
}

add_filter( 'searchwp_extra_metadata', 'my_searchwp_index_wc_order_number', 10, 2 );