This can be found on letters from Companies House to your company. You can look it up on Companies House website by searching for your company name.

The Company Authentication Code was sent from Companies House to the director/s when the company was registered. If you cannot find it, you can request a reminder from Companies House to be posted to the company's registered address. This will take about 7 days to arrive. We can only submit the Confirmation Statement when you have both the Authentication Code.