Ideally, you don't start building any software until you have a validation. "Soft" validation would mean having people that want to try it, "hard" validation would be preorders. A middle ground might be what Buffer did [1], where they added a fake preorder screen just to see whether people will click on it.

Beware that any kind of "soft" validation may evaporate once you'll try to charge money, so start charging sooner.

[1] https://blog.tally.so/how-to-validate-your-idea-with-tally