setUserId method
Set a custom user Id.
If your app has its own login system that you want to track users with, you can set the userId.
amplitude.setUserId('user Id');
Implementation
Future<void> setUserId(String? userId) async {
Map<String, String?> properties = {};
properties['setUserId'] = userId;
return await _channel.invokeMethod('setUserId',
{'instanceName': configuration.instanceName, 'properties': properties});
}