There are (2) ways to cut left-handed threads on a Haas lathe.
First, you can mount left-hand tooling to your turret and run the spindle in reverse to cut left-handed threads. Left-hand tooling has the insert facing the operator when the tool is in the turret.
You can also cut left-handed threads with right-handed tooling. To do this, you must program the thread to cut away from the spindle head (in the Z-positive direction). Note that with this method, you need to create an undercut at the chuck end for the thread cutter to enter the thread start point.