From here, you can:
1. You will need to be logged in to BMail and no other Google accounts in order to manage your seat reservations.
2. The first time you use the reservation link, Google may display a warning asking for permission to access your account. The reservation software uses your email address for the reservation, so please say yes. Otherwise, you will not be able to reserve a seat. After you give permission, it's sometimes necessary to reload the page in order for the reservation form to appear.
3. Microsoft's Edge web browser doesn't seem to work with the reservation page. Google Chrome, Mozilla Firefox, and Microsoft Internet Explorer do work.