Source "Keyboard"

This source reacts to pressing or releasing a key on your computer keyboard. It emits a value of 100% when the key is pressed and 0% when released.

In order to set the key, simply click the Learn button and press the key of your choice.

In addition to the key label, ReaLearn might show some warnings regarding the portability of your keystroke. This helps you to avoid keyboard shortcuts that don’t reliably work cross-platform (in other operating systems) or on other keyboard layouts. You can ignore portability warnings if you use just this operating system and don’t plan to share your keyboard presets with other users.

This only works if Computer keyboard is enabled in the Input menu.
If you hold a key, it will not keep firing. This is by design! Use Fire after timeout, keep firing (turbo) instead.
Key combinations are not supported. This is by design! Use Conditional activation instead.