Sync Service is using Auth SDK to provide authentication and authorization. When user login in Auth SDK user principal could be used to retrieve token. SyncSDK can use this token to perform requests to the sync backend
To do that please follow instruction depending on the platform
For IOS: https://github.com/aerogear/aerogear-ios-example-apps/blob/master/Memeolist/Memeolist/AppDelegate.swift#L19-L20
For android: https://github.com/aerogear/aerogear-android-example-apps/blob/master/memeolist/app/src/main/java/org/aerogear/android/app/memeolist/ui/MemeolistApplication.java#L29

Instruction
1. Setup Auth as in auth docs
2. Connect Auth to Core (That will be used by sync)