Registration is typically handled automatically by when an iPhone is connected for development. If you need to find the ECID manually, you can use these methods: macOS Finder/iTunes : Connect the iPhone to a Mac or PC.
Activation locks are tied to Apple ID accounts and iCloud, not directly to ECID. If you see an activation lock error referencing ECID, it‘s likely a misinterpretation — the actual issue is usually related to Apple ID credentials. Apple‘s official support channels require proof of purchase (receipt, box) to remove activation locks, not ECID registration . ecid register iphone exclusive