Basically, the key is keeping track of the "previous" position and comparing it to the "current" position. In the tutorial, the object being flicked is the "TouchFollowMe" object, which in your case would be your projectile (and your sprite would not be invisible). You also don't need the scrolling part, so you wouldn't be using the Scroll To behavior. You may or may not need the inertia, which can be modified by either changing or removing the lerp() function.
An alternative solution still utilizes the start and end positions of a touch event, but you can simply store the origin coordinates in variables, then use the angle() and distance() expressions with your start and end coordinates to set the properties of your projectile.