How do I add a device that is part of iCloud to find my iPhone??

On your iPhone 8, open Settings ➔ Apple ID (Your Name at Top) ➔ Find My : Here, set Find My iPhone to ON and Share My Location to ON. Now it will be visible to your devices that are signed on to iCloud with the same Apple ID.

Axel F.

On that iPhone 8, you tap settings, your name at the top, then Find My and turn it on. That is the only way to make a device trackable.