I'm confused by your question.
If you need to mark your previous iPhone as stolen, you can access https://www.icloud.com/find/ or open the Find My app from any of your other Apple devices.
If you've opened Find My from the link above, you can click "All Devices" at the top and then click on your iPhone. Once you've clicked on the right device you can click "Lost Mode"
On macOS, you'd open the Find My app, click "Devices" at the top of the application, and then right click the iPhone you lost. A menu will appear and you'll want to click on "Mark as Lost..."