Crossplatform Beta Key is a technical Items. It is not possible to obtain it anymore.
As of the Crossplatform update, the item is not required to be able to use crossplatform feature.
Crossplatform Beta Key used to be useful only for mobile users. It was given out during Podcast with a code.
When used with Berzerk Pals cloud save, it allowed players to play freely on Zombidle.com, Mobile, and on Steam.
It was given out during Podcast with the code: ScreechingSulkyBoomrat, but the code has expired.