Not yet released for the Pixelbook.
It's been several days and V76 has still not been released for the Pixelbook. I wonder why it takes so long to be released on Google branded Chromebooks?
The update became available for the Pixelbook the evening of 8/16 and I installed on 8/17. Enabled the virtual desktops flag. Observe that the touchpad gestures to switch between are not yet working, need to do a three finger swip up or press the multitask display button to display the desktop options, then click the one I want to switch to.
Chrome Unboxed provides a good description of how you can use Virtual Desktops when the full functionality is available. (It is not fully available in V76). page 
If you want to have browser windows in each virtual desktop you want to first load Chrome as a new window in each desktop, otherwise when you try to access an URL it will witch to desktop in which Chrome is currently running.
If you press Alt+Tab you will see all running apps across all desktops not just the apps running in the current desktop. I am not sure if this is the expected behavior for virtual desktops.
Google released Version 76.0.3809.136 that became available for the Pixelbook sometime during the weekend of August 31, 2019. I have not seen any information about what is in this version, but someone did post on Reddit that it was a bug fix.