If you experience an authorization error from Google's Advanced Protection Program when signing up, you may be able to whitelist Shortwave as a trusted app via the Google Admin Console.
If you are the admin of a Google Workspace account, you can whitelist Shortwave as a trusted app:
Go to your Google Admin Console
At the top of the Configured apps list, click Add app
Choose OAuth App Name or Client ID
Enter the app's name: "Shortwave" and then click SEARCH
From the list of search results, click Select for the app that you want to manage
Click the top check the box to select all OAuth Client IDs and then click SELECT
Choose: Trusted and then click CONFIGURE
After that, you should be able to sign into Shortwave. If you run into any trouble getting set up, please email firstname.lastname@example.org for assistance.
It is currently not possible to use Shortwave with a personal Gmail account enrolled in the Google Advanced Protection Program. To use Shortwave, you must un-enroll from Advanced Protection.