XOGO supports portrait mode on most devices simply by changing orientation within XOGO Manager.
However, some devices don't support automatic portrait orientation. A the time of this writing Amazon Fire TV falls into that category. In order to run your content in portrait (vertical mode), simply design it in vertical format in your favorite editing program. Export to your desktop, and then rotate the content either 90 (if the top of your TV is to the right) or 270 degrees (if the top of your TV is to the left), save, and load into XOGO as normal. This quick tutorial shows you the process.