http://www.pcgamer.com/call-of-duty-wwii-open-beta-preloading-has-begun/

The Call of Duty: WWII multiplayer open beta gets underway this weekend—September 29, to be precise—and those of you who want to avoid wasting time when ye olden shooting begins can start preloading it now on Steam. 

The open beta client is separate from COD:WWII on Steam, and is roughly 14GB in size. "Beta participants will experience the all-new War Mode where players compete against each other in objective-based, team gameplay," the standalone Steam listing says.