The documentation for the external goma did not cover Windows but I gave it a try anyway. Does not seem to let me in though:

L:\src\goma_chromium\src>l:\src\goma\goma_auth.py login
....
Do you agree to our data usage policy? (y/n) -->y
You have agreed.
Please visit following URL with your browser, and approve access:
....
Please input the code: *****************hunter2**********
Login as bratell@opera.com
Current user is not registered with Goma service. Unable to use Goma

I guess it might be because I had already authenticated on my Linux machine too. Or because the external goma beta doesn't exist for Windows. I'll stay on Linux for now.