I would like to know if it is possible to send a private message to an author.
There’s no way to do so directly through the software. Authors have the option to reveal or hide their e-mail address on a per-story basis, though, so if you don’t see an e-mail address on the story you’re reading, have a look at other stories by the same author. Sometimes, authors change their minds or whatever and so you’ll see their e-mail on one story but not another.
You can also look around the forums or check out the Discord server (linked in the title bar of the main site) to see if the author is using the same name in both places. Typically, they do, so you can contact them that way.
You might be able to find them on the discord app by asking if that author is there. The author might reveal themselves there. But this is strictly voluntary.
I’ve seen an instance where a reader left a comment in the comments section asking to contact the author privately - and the author responded with a solution. So if all else fails, put a comment in the comment section.
Adding a comment to a story will usually create a notification email for the author, so you can try to ask him for a way to contact him.
If you’re using the forum, you can receive and send PMs in the forum. If the author does the same, you can use that to write PMs without the need of any EMail addresses.