For Microsoft Certified Trainers, an email is sent with a link to a form to request a voucher for the beta exam.
For everyone else, a beta code is published on the Learn blog.
The blog post and email are normally sent when the exam is released. SC-100 is released on April 7th so check the Learn blog in a couple of day's time.
The email to MCTs for SC-100 beta went out early this time, no idea why. The requests for beta codes are processed manually as the MCT details have to be manually validated; it can take a week or so for beta codes to be emailed out.
The purpose of beta is to validate the exam and to ensure it is at the correct level. Microsoft needs around 300 people to take the beta exam so this is why there is a strict limit on vouchers.