SimInput

SimInput is a GameMaker extension for Windows that lets you simulate mouse and keyboard input. This can be used for interfacing with your game or even other programs programmatically, as if you were clicking or typing manually. Useful for automating testing of your game, or developing macros for other software.

Some extra features include being able to change system volume, play and pause media playback, and check for mouse inputs even if the game window isn't focused.  

View the online documentation here.