This repo contains the code for a Teensy which has buttons via an analog input connected. When pressing a button it sends a key combination in the form of CTRL+SHIFT+ALT+WIN + <letter> and it starts with the letter a (= button 1, defined in the code).
The buttons are differenciated by voltage division using resistors, see the Fritzing schematic. You can use the Excel sheet to get the resistor values you need. The voltage values shouldn't be too close together so that you can differenciate each button.