Using Safari Extension on Setapp
To use the Safari Extension with Setapp's Secrets, you'll need to install it from the Mac App Store.
Note: In versions prior to 4.1.6, Secrets for Setapp did include the Safari extension with the app. But this required enabling "Allow Unsigned Extensions" in Safari even though the extension is properly signed and sandboxed. This was an unexpected difference in behavior between Safari Web Extensions (used by Secrets 4) and Safari App Extensions (used by Secrets 3). While both types offer similar capabilities, Apple has apparently decided that Safari Web Extensions will only work out of the box if installed from the Mac App Store.