To add a number, a mention or anything else on your invoice, you can do so while entering your payment details, it's called "Special notes on invoice (e.g. PO)". It's the last part of the payment details, just below the section of the VAT number.
If you forgot to do it or couldn't do it before confirming the booking, you need to contact us directly. You can do so by phone (the number of your assistant is written at the bottom of the page) or by email at email@example.com.