Check the help file under the RGSS Modules, Input.
The basic syntax is:
Input.trigger?(Input::CONSTANT)
Input.press?(Input::CONSTANT)
Input.repeat?(Input::CONSTANT)
You can choose 1 of them, and use the constants defined. A, B, C, L, R, X, Y, Z, LEFT, RIGHT, DOWN, UP, CTRL, SHIFT, F5, F6, F7, F8, maybe a few others.