Unfortunately the short answer is that there is no trigger for 'new post' in a Slack workspace (not least because of the overhead).
Even working around it isn't for folks like me who barely know any code, as you will likely need to create a SLACK App.
That said, it's not totally daunting.
Some of my work around revolve around not reacting to every message, but rather offering workflows which update something on a connected service, then you can fire your flow based upon that.