Lock iPhone with passcode

Mark As Lost. This will remotely lock your device with a passcode and you can display a custom message with your phone number on your missing device's Lock screen. It will also keep track of your device's location. If you added credit, debit, or prepaid cards to Apple Pay, the ability to make payments using Apple Pay on the device is suspended when you put your device in Lost Mode.

You can not do this if the device is no longer in your possession. If you want to Mark The iPhone As Lost to where no one can access it go to Find My > Devices > Choose Your Device You Want To Lock > Then Click The Activate Button Under The Mark As Lost Icon.

You can’t change a passcode on a phone that is not in your possession.