To find your idevice udid, please follow below steps.
Note: To Connect Apple TV 4th Gen to PC or MAC, you need a USB C cable, thats the only way to install apps on Apple TV 4 at this time.
Step 1: Connect your Apple TV to PC or Mac using USB C Type Cable.
Step 2: Open iTunes & Select your Connected Apple TV.
Step 3: Click on Serial Number, it will show UDiD.
Step 4: Copy it by clicking Control + C (Windows) / Command + C (Mac) & paste its somewhere safe.
You will need the UDID to register your Apple TV 4 With IPwnStore