I think you might be getting confused on where the username and password is being required. TestFlight requires an Apple Developer Program membership, thus you have to sign in to TestFlight with an Apple ID that has that subscription. There shouldn't be any requirement for the app that you are testing to collect login information for any reason (unless that is part of your app's capabilities, which it looks like is not the case).
Topic:
App Store Distribution & Marketing
SubTopic:
TestFlight
Tags: