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:

That's it!