Click the green flag, then follow the directions on the screen. The hitbox will become invisible when the green flag is clicked and the project is running, but you (and other sprites) can still interact with it, just like the hitbox for any standard video game!
The project is really basic, so no need to credit me excessively for using this technique in your own projects But if you take direct inspiration from using this project, just slip in a "credit to Alextorrez6" if you can. Much appreciated, and keep scratching on!
