Check this article about the Activation Lock feature:
Find My iPhone Activation Lock - Apple Support
Find my iPhone has to be activated on your device before the device is lost. You can't activate is later, because the person who found the phone might already set it up with a different Apple ID.
If you ever misplace your device—whether it’s just lost or you think it might have been stolen—you should put it into Lost Mode immediately with Find My iPhone.
Lost Mode locks your screen with a six-digit or four-digit passcode and lets you display a custom message with your phone number to help you get it back. You can also remotely erase your device if needed, and your custom message will continue to display even after the device is erased.
While Activation Lock works in the background to make it more difficult for someone to use or sell your missing device, Lost Mode clearly indicates to anyone who finds your device that it still belongs to you and can’t be reactivated without your password.
To put a device into Lost Mode, sign in to icloud.com/find from a Mac or PC or use the Find My iPhone iOS app.
You can learn more about what to do if your device is lost or stolen.
copied from: Find My iPhone Activation Lock
