The Hitman PC Beta has been re-scheduled to start at 17:00 CET on February 19th 2016 and end on February 22nd at 17:00 CET worldwide. (Pre-loading of the HITMAN Beta is scheduled to start at 17:00 CET on Feb. 17th.)
You gain access to the beta by pre-ordering the game. You can find more details about what the beta includes and how big it is over here.
Here’s the trailer for the beta, which introduces the setting for the mission. It’s a prologue for the game, and so separate from the Paris mission which will form the majority of the game’s first episodic release.