From the “My Books” tab, select the book you want to add the bonus to. Before you publish the book, you’ll see the “Standard, Quick, Quicker, Fastest, or Custom” options in the bar above the token cost to list your book. Select which of the 5 options you’d like to use and then publish your book.
If your book is already published, then you have to unpublish it before changing the bonus options.
You can learn all about the bonus system from this help article.
