I believe that login request is coming from the ArcGIS Online service itself. The license key is for licensing the app. You can separately provide credentials programmatically when you load the layer, table, etc. See "Connect to secured content and services on ArcGIS Online" here: Access the ArcGIS platform—ArcGIS Runtime SDK for iOS | ArcGIS for Developers
Alternatively, you could share that content publicly, if appropriate; then, no need for credentials.
Hope that helps.