Final Draft is a preferred file format and the only screenwriting software with an authorized agreement with the WGAw online registry service.
Go to File > WGA (Windows) or File > Register Script (Mac). This will take you to a web page on the Final Draft site (make sure your computer is connected to the internet).
Please note that you are registering your script with the Writers Guild, not with Final Draft, A Cast & Crew Company. All inquiries should be directed to the Guild.