It seems like this should be a very easy question to answer, but I can't find the answer anywhere. Going to https://mydomain.onmicrosoft.com/owa seems like the most logical answer, but I get a "This Page Can't be displayed" error.

How do I access OWA on my Office 365 based Exchange account on the default domain? (mydomain.onmicrosoft.com)

http://outlook.com/owa/yourdomain.mail.onmicrosoft.com

It will redirect to the correct URL ...

or simply:

https://outlook.com/yourdomain.com