CloudKit in iOS
Registering app for use with CloudKit
What you need is to get an entitlements file so the app can access your iCloud and write records using CloudKit.
Follow the steps to grant access to iCloud from your app:
1- Select the project in the Project Navigator, and then open the General tab.
2- In the Identity section, set your developer Apple ID to the Team dropdown menu. (If it is not available, add it in Xcode menu -> Preferences -> Accounts.