[mdbennett]

Zapier Support here. The most common way I see people do this is by starring Slack messages they want to add as Todos. Here's a template that uses Todoist, although there's no shortage of alternatives on that side of things: https://zapier.com/app/editor/template/1580

Outside of that, you can build a zap that triggers on any message and use a Filter step to make sure only messages that meet your criteria(e.g. your keyword) move on to the action: https://zapier.com/help/how-get-started-filters-zapier/

If your ultimate goal is to manage the todo list from inside Slack, there's not a great way to do that with Zapier. Personally, I manually add/delete items in my 'You' channel to get to that type of functionality.