Arduino nano as HID Keyboard
Get yourself a Arduino Leonardo, Micro or Pro Micro (or Due, Zero, M0). Those can emulate a keyboard.
Start with reading the Arduino Keyboard Mouse reference.

The Arduino Nano can not use the Arduino Keyboard Mouse library.

Since many years, there is a library called "V-USB" that requires some extra hardware and makes it possible for a ATmega328p microcontroller to act as an USB device. It is not something for a beginner.

A few years ago, Arduino has changed the way the USB is used. The NicoHood HID library makes use of the new possibilities. It makes it possible for example to have extended features for a USB keyboard, like the media keys.

For a normal keyboard and mouse, the Arduino Keyboard Mouse library will do. It will be perfect for a ignition button. But you have to buy one of those boards (Leonardo, and so on).