Crossplatform Beta Key is a technical Items and is not acquired by gameplay.
Crossplatform Beta Key is useful only for mobile users. It was given out during Podcast with code.
It is used alongside the Berzerk Pals cloud save to enable players to play freely on Zombidle.com, Mobile, and upon release, Steam.
It was given out during Podcast with the code: ScreechingSulkyBoomrat, but it is expired now.