In the US, the beta program is available for T-Mobile and Sprint customers and to those using an unlocked Galaxy S9 or S9+. It’s not known whether other carriers will run the beta test as well.
It has a big size, it is 1790.17MB. The Software version is G960U1UEU3ZRK9 and G965U1UEU3ZRK9 for the Galaxy S9 and S9+ respectively.
If you haven’t rolled into the One UI beta program yet, you’ll have to do so in the Samsung Members/Samsung+ app, where you should see an option for enrolling in the beta program after tapping the “Notices” button. Afterwards, you can download the newest Android Pie build by going into Settings > Software updates and tapping on “Download updates manually.