By default, when you hit the “Post” button your playlist gets saved. It will get picked-up as soon as you start the “Playlisty” app. This is the way that Apple says it’s supposed to happen.

In case you find this laborious and would prefer to go straight to Playlisty when you hit the “Post” button, there’s a switch (at the bottom, under the Post button). If you change this from “Import Playlist -> Later” to “Import Playlist -> Now” then the Playlisty app will start as soon as you hit “Post”.