How to deeplink directly to build in Testflight App

For giving our tester a direct link to download the correct version from testflight we would like to somehow get the build id. The emails sent by Apple that a build is ready to test contains such a link: https://testflight.apple.com/v1/app/[appId]?build=[buildId]. When trying to use the official App Store Connect API one can find a buildId, unfortunately that one is a UUID, not a normal upcounting number as the email contains as buildId. Replacing the buildId with that UUID from the API also doesn't work. Would anyone know how to get those testflight buildIds without intercepting testflight traffic with Charles Proxy or similar?