The event is not cancelable.

The event does not have a result.

Importing the class

Link to importing-the-class

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

import crafttweaker.forge.api.event.block.piston.PrePistonEvent;

Extending PistonEvent

Link to extending-pistonevent

PrePistonEvent extends PistonEvent. That means all methods available in PistonEvent are also available in PrePistonEvent