splashingpixels.com on "[Plugin: WooCommerce - excelling eCommerce] custom order data into database"

So do this:

add_action( 'woocommerce_checkout_update_order_meta', 'nfp_add_meta_webcode', 10, 2 );

function nfp_add_meta_webcode( $order_id, $order_posted ) {
	$unique_code = 	$_SESSION['clientwebcode'];
	update_post_meta( $order_id, '_webcode', $unique_code) ;
}