I can’t find any docs on how to use Quick Add Magic, only forum posts here and this announcement: What's new in Vikunja 0.18.0 | Vikunja
After quite a while I now figured out that I have to enable it, even on https://try.vikunja.io where I expected it to work out of the box.
Furthermore, if Quick Add Magic is off and there is no default list, adding tasks from the home screen seems to be impossible.
In fact, I haven’t found any user documentation except for the feature overview page, making it quite hard for me to get into all the powerful features of vikunja.
And by the by, the installation docs mention the app is a fine substitute for the web frontend, which does not seem to be the case