[stumped]

dropbox may be easier

moon reader already has a feature called dropbox sync which writes all your annotations to a file called name-of-book -author-of-book.an in a dropbox subfolder.
e,g,
pc path\Dropbox\Apps\Books\.Moon+\Cache

you just have to give moon reader permission to use your dropbox account and enable the dropbox sync feature in misc options.
Then work out how to extract notes from those .an files if you want to work with them on PC, rather than just store them.

( I am not sure if you also need to install the dropbox app on your fire tablet but to do that you get it directly from dropbox.com via silk browser ( it is not in the amazon app store) , then allow install from unknown sources in fire security preferences