All you have to do is click the big blue button and it will have you bind keys one at a time. You have to use lower case and if you want to bind an arrow key you use lower case with a space (e.g. up arrow). If you don't enter anything, then shift alt and ctrl will be bound to that action. This project uses "hacked" blocks in order to have the binding work, but all you have to do to use these in your own project is backpack the blue lines that say "when key Left Bind is pressed", or similar and change the variables with a right click.
Thought of the idea today, so I quickly put something together. Uses the engine I made for Platforming Pals 2 (PPTX engine). To make the hacked blocks I used 7-zip and some other stuff, you can just backpack the hacked blocks if need be. I also "remade" the Mario sprite in scratch, didn't know what else to use.