As part of your invitation to a beta, you should have been provided a link to a particular section of the Age of Empires forums for reporting bugs and providing feedback. Depending on the exact beta, this could be a closed forum or sub-forum, or a particular thread.
If you are unsure where you should post, please feel free to ask a moderator on the Age of Empires forums, reach out on the game support channel in our Discord, or email AOEInsiderSupport@microsoft.com to ask where you can post about that beta. Remember that most betas are confidential and under NDA, so please get confirmation on where you should post before you actually post.
We may also periodically email out user surveys about your experiences in a beta. These are super important as the information gathered is used to drive changes to the game and the Age of Empires Insider Program at large.