To simplify what @Hexcede is saying, and to provide an extra functionality for those without a MIDI keyboard:
You need to map MIDI keys to regular letters on the PC keyboard via a program on your PC, so Roblox just sees regular computer keys.
One way to do this, that several music composing programs use, is to map the white keys to (ASDFGHJKL) and use parts of the top line (QWERTYUIOP) for the black keys where they sit between the keys below.
That makes it simpler than picking random keys, and it also helps your players who don’t have midi keyboards as they can play in a relatively intuitive way.