This topic was discussed in the past but no solution was provided. Here is the link to the old thread
Problem is, as soon as testflight processes the build, it sends out notifications to the testers. I would like to add test notes before the testers gets the notification. Else they get a notification with empty release notes.
we currently have a task to make this in our backlogs, I don’t yet have an eta for this but will be available shortly! we will notify you in that case!
I’ve been looking at this recently The only complication though is that Fastlane requires the AppStore Connect processing to complete in order to add the “What to Test” field. From my testing this can easily push the build time over the 10 minute limit, so if you’re on the Bitrise Free plan this might be an issue