If you got this version in the way provided by the author, then you already have access to this beta version of the game. If you took it from a third-party site, this event is not there.
It is worth remembering that the owners of sites that host beta versions of games from Patreon usually do not strive to update beta versions. Therefore, their users usually play on versions with a bunch of errors. As it was, for example, with 0.70.001.
Here, on this page, this event will become available when version 0.72 becomes stable, because this event is already released as part of version 0.72.