Currently, you can't login to morgue directly with an SSO-only account. However, there is a workaround:
First, login to the Backtrace UI as usual. Then, navigate to https://youruniverse.sp.backtrace.io/api/session and copy the token in the JSON data that appears.
With this token, you can login to morgue by issuing the command:
morgue login https://youruniverse.sp.backtrace.io --token=..where token is the token you copied from the api/session link above.
Note that this token may be reset upon server restart or maintenance, so if you receive messages about invalid tokens or authentication, repeat this process.