Hi!
If your iPhone is covered with Theft and Loss AppleCare+, you can sign in with your Apple ID and submit an iPhone replacement request. Use the Find Me app or go to iCloud.com/find to mark your iPhone as lost. Do not remove the device from your account until your request has been fully approved.
If you have AppleCare+ with Theft and Loss, do not remove your lost iPhone from Find My or your Apple ID until your request is approved. Go to appleid.apple.com to remove the missing device from the list of your trusted devices.
Applecare+ with Theft and Loss may not be available in all countries and regions. I recommend that you contact Apple Support for more information about warranty coverage in your country.
I hope I helped you!