I would go full text-mode (Markdown), Keynote or Ludus[1] [1]: https://ludus.one/. - Source: Hacker News / over 1 year ago

Marvelapp.com — Design, prototyping, and collaboration, free plan limited to one user and project. - Source: dev.to / 4 months ago

At this stage your main goal should be to prototype it and test it with people to validate the idea. Or at the very least have something people can look at and respond to. Don’t worry about building a coded and working version yet. Start with a clickable prototype which can be built using design tools. Most people use Figma these days but if you’re just starting out you could use something like Marvel, which is... Source: about 1 year ago

Marvelapp.com — Design, prototyping and collaboration, free plan limited to one user and one project. - Source: dev.to / over 1 year ago

Hi, I am doing research on some of the user testing tools out there like lookback.io, Marvelapp.com, maze.design, usabilityhub.com, userbrain.net, usertesting.com, userzoom.com. I would like to know about your experience. Source: over 1 year ago

As far as I can remember, I saw https://marvelapp.com/ doing it to add a prototype to the homescreen. Source: about 2 years ago