Turn on your Fire TV device and click on the Search icon.
Type Downloader using the remote.
Click Download to install the Downloader app from the Amazon App Store.
Go to Settings → My Fire TV → Developer Options → Install Unknown Apps → Downloader → Turn on.
Open the Downloader app, enter https://www.boxseats.tv/download/ and install the Box Seats app.
Once installed, select Open to launch Box Seats and sign in with your subscription details.