Try to have several such conversations. Then you can build a "sales" demo - it can be a presentation with slides, or it can be quick demo-prototype (it's possible to develop it in 1-2 weeks in some cases). Then you can setup a meeting with the most friendly potential customer from previous conversations and do the first demo run with them, then refine demo based on feedback and do it again
If you find that a company is spending skilled time (translation:money) on a crappy internally-cobbled together verson of your product, even better.
So if you expect to get signups mainly through your website, you need to test that channel as early as possible.
Eventually you'll be getting hundreds (millions?) of signups through your website. So getting just a dozen for your beta list should be easy. If it's not, then you need to either pivot or find a better channel.
But if your website isn't the way you expect to gain users, then try a different channel.
And I don't think Slack is a good example since Campfire and Hipchat were already in use many years earlier.
To add on further, we wrote an article about idea versus execution and it dives into this towards the end. Here is the link: https://medium.com/waccaler/idea-is-only-a-small-piece-of-th...
I do agree with afarrell's comment that there was already existing solutions in slack's market. Slack just did it better and met a lot of needs that were not met. Also, slack is a marketing machine, which can explain a lot of their traction as well.