How to create promo codes for your iBook edition through your Apple iTunes Connect account:
1. Login into your iTunes Connect account
2. Choose ‘Manage Your Books’
3. Select ‘Promo Codes’
4. Send your reviewer the promo code which they will be able to ‘redeem’ through the iTunes store.
Here’s some screenshots:
Here’s more about Apple’s iBooks promo codes (from Apple’s FAQ):
What are promo codes?
Promo codes can be distributed to provide free copies of your book for promotional or publicity purposes. Once you have distributed a promo code, the recipient can redeem it in the iBookstore for a free copy of your book in any territory where the book has been marked Cleared For Sale or Not Cleared For Sale.
How do I get promo codes for a book?
You may need to enter into a separate Promo Code agreement to request promo codes for a book, and so the requestor must have the Legal role to request promo codes. You can request a maximum of 50 promo codes per title. Promo codes are valid for 4 weeks from the date they are requested. To request promo codes, locate the book in the Manage Your Books module and click Promo Codes. Enter the number of codes you would like to request and click Continue. After reading the contract, check the box to agree to the terms and click Continue. After agreeing to the contract terms, click Download to download the promo codes.
Why don’t I see the Promo Codes button?
Only users with the Legal role can request promo codes. The Promo Codes button will only display for books with a status of On Store or Ready for Store in at least one territory. An admin user can assign the legal role to another user via the Manage Users module.