You were right - that was a newline issue. I tried the same API call from Postman, and it worked - a new staged user was created and a topic on his behalf.
But: no email was sent to this user’s address! I also posted a comment in the topic - no email notification either. Is there a setting that prevents this? I tried to find one, but I couldn’t.
UPD: There was a indeed a setting that prevented sending post notifications 
so now I have two different questions:
- Why there wasn’t an email informing the staged user that the topic was created and sending him the link to it?
(I would also add some instructions about how to log in to the forum if I knew, which template to edit.) - Why there’s no link to the topic in the post notification emails? Also no unsubscribe link.
The template seems to be the same - “user_posted”.