WooCommerce: Save custom checkout field

woocommerce-save-custom-checkout-field

add_action( 'woocommerce_checkout_order_processed', 'prefix_save_field_on_checkout', 11, 2 );
function checkout_order_processed_add_referral_answer( $order_id, $posted ) {
	if ( ! isset( $_POST['checkout_field_name'] ) ) {
		return;
	}

	$order = wc_get_order( $order_id );

	// WC < 3.0
	update_post_meta( $order->id, 'order_meta_field_name', wc_clean( $_POST['checkout_field_name'] ) );
	
	// WC > 3.0
	$order->add_meta_data( 'order_meta_field_name', wc_clean( $_POST['checkout_field_name'] ), true );
	$order->save();
}