You can’t opt in, but credits to @LucasTutoriaisSaimo this is what you can do.
Right Click on Roblox Player, and click properties.
Once your in you’ll get this panel, renamed RobloxPlayerClient to RobloxPlayerBeta
Then press Apply, then OK. Now don’t run roblox by clicking on the play button on a game. Double-Click the desktop icon that you were able to get the properties panel on.
It should open the new roblox, but if it doesn’t, put " infront of the C:\ and put " at the end of ‘exe’. Then you see where it says -app? Put --app instead.
That should work! There’s currently not a way to do it on mac or linux.