Note: “pause” isn’t the way to do this as others might suggest. (And, of course, they have.) And, I wanted to write a long message about how this is a terrible user experience, but I’m just going to leave it at that.
If you want to pop a modal the first time someone encounters a certain feature, just do that (via action “Show Popup”). But just know that most of them, being smarty-pants, will dismiss it. That’s all you can do, really.