To find your idevice udid, please follow below steps.

Note: To Connect Apple TV 4th Gen to PC or MAC, you need a USB – A to USB C cable, thats the only way to install apps on Apple TV.

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 UDiD for various things. So make sure you have it with you.