Here are the steps provided by Google to join the Chomecast Preview program:
"1.From your Android or iOS device, open the Google Cast app Chromecast cast icon.
2.Tap the DEVICES tab to see your available Chromecast devices.
3.Scroll to find the device card for the Chromecast you'd like to enroll in the Preview Program.
4.In the top right corner of the device card, tap the device card menu .
5.Tap Device Settings.
6.Tap Preview Program. If you don’t see Preview Program, we aren’t accepting new members at this time. Please continue to check back as opportunities become available.
7.Choose whether to receive email notifications by moving the slider to the right or left. Note: We recommend allowing email notifications so you’ll know when new updates are pushed to your Chromecast device.
8.If you opt-in to email notifications, you'll be asked to sign into your Google account if you haven’t already.
9.Review the contents of that page, and tap Join Program.
10.Review the contents of the page, then tap OK, GOT IT. "
For additional info checkout the support page on the program https://support.google.com/chromecast/answer/6343937?hl=en