We don't support paid pre-orders.
However, we do give customers the opportunity to sign up as "Interested Readers" and be informed when you publish your book for the first time.
That feature is automatically enabled and is displayed prominently on your book's landing page, to the right of the book cover.
Here's how this works: people can tell you what they would pay for the book, and they can choose to share their email address with you. If they don't want you to see their email address, they can still give their email address to us, so that they will get an automatic email from Leanpub when you publish your book for the first time.
Here is what the form looks like on the landing page for an unpublished Leanpub book:
When people sign up, you will see information about that if you go to your Book Tools menu, select Community and then Mailing Lists , and then click on View to the right of "Interested Readers."