You should connect your Splash to your external flash and set the last parameter (Trigger) on the app to Flash.
In this mode, you should use your camera at long exposure in a dark room. If you already have a camera connection cable and connect it to the camera, Splash triggers your camera to start long exposure, then drops water and finally trigger your flash to stop the motion. If you don't have a camera connection cable, you should start long exposure on your camera manually and then press start on the app.
Please set your Flash to M mode.