Well, there are 2 ways now it seems. You can participate via the Ubisoft Account management https://account.ubisoft.com/beta-access under the tab "Beta Access"
It should list what Platform you took. Depending on your selection this is where you can change to PC, PS4 or Xbox always excluding the currently selected one.
Second "classic way":
If you did select a Platform in 2018 over bgegame.com it should be populated too. If this option still exists for the redesigned website beyondgoodandevil.ubisoft.com then you should end up with the same result.
If you are not opted in you are very likely able to do it on this very Beta-Access page as I could do for other Betas. Please keep in mind this registration is essentially a thing since 2018 and we do not know any concrete details about releases.
Furthermore this website tells you if your current Alpha/Beta/EarlyAccess to any game is under NDA (Non Disclosure Agreement) in case you want to know that. And do not share the contents of this website. It contains your information.

I hope you got all your answers regarding Beta Access.
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