Where to find an audience for my Slackbot to get pre-launch and beta feedback?

I'm developing a Slackbot https://www.productivebot.com/blog/how-to-use-slack-to-manage-your-approval-process for streamlining approvals (expenses, invoices, holidays, etc). I know there is always a need for approvals, especially in larger teams, but without spending too much time on development I would like to get some feedback on the idea and have potential users ready who could actually become customers ready for beta testing and willing to pay.

So without launching on the Slack app market (it's possible to add a Slack app that's not on the official store), how can I go about finding my audience? Where do I start? Reddit, Twitter, LinkedIn? Who do I target? CEOs, hiring managers, accountants?

Any ideas will be great.