Here is the section of the Markua manual with instructions for making footnotes:
https://leanpub.com/markua/read#leanpub-auto-footnotes-and-endnotes
Here's an example of how to do it:
# Chapter One
This has a footnote[^thenote].
[^thenote]: This is the footnote.
This will produce the following result (this example is from a PDF version):
You don't have to type [^thenote]
; this "anchor" can be anything you like, and should be different for each footnote. For example, you might do:
# Chapter One
This has a footnote[^1].
[^1]: This is the footnote.
or:
# Chapter One
This has a footnote[^a].
[^a]: This is the footnote.
The "anchor" is not visible to readers. So, you can use numbers in the footnote "anchors" if you want to, but they will not affect the displayed footnote numbering in the book, which just happens in the order the footnotes appear in your manuscript.
You can see the footnote settings for your book on your book's Theme page here:
leanpub.com/YOUR_BOOK/settings/theme
...making sure to replace YOUR_BOOK with your book's unique web address.