Here are the instruction on how to boot tethered your Apple TV using SeasOnPass
Download the latest version of SeasonPass from here.
Launch Seas0nPass by double clicking its icon.
Select Boot Tethered from the main menu.
Connect the Apple TV to the computer via USB, plug it into the power outlet, and hold MENU and PLAY/PAUSE on the Apple TV remote for 7 seconds.
Seas0nPass will detect the device in DFU mode and begin the boot process.
Once the tethered boot is complete you can remove the USB cable and connect the HDMI cable to begin using your jailbroken device.