To add a "Dedication" page to the beginning of your book, go here:
...making sure to replace YOUR_BOOK with your book's unique web address.
You can also find a link to the Dedication page from your book's Overview page:
On the Dedication page, you can enter your text here:
Once you're ready, click the blue "Update Dedication" button.
The next time you create a preview of your book, you will see a page at the beginning of the book, with your Dedication text, like this: