Will it be possible to have a userID in the app created. This is required to track unique user using different platform with Amplitude.
User ID: This identifier is configured by you. Many products use a username or an internal unique identifier to track their users. To set a User ID, use the setUserId method outlined in our SDKs. IMPORTANT NOTE: If the User ID gets changed, then this would create a new user. We recommend not setting the User ID to something that may change (e.g. email address).
After gathering the Device ID and User ID, Amplitude formulates an Amplitude ID. The general rule is that we will prioritize using a User ID over a Device ID when assigning an Amplitude ID. Amplitude uses this Amplitude ID to identify unique users. With the exception of special circumstances, an Amplitude ID will always be prioritized over a Device ID or a User ID. See the examples below for different scenarios.