Flutter Segment: Merge events for single user

Question

I am using Segment for analytics that is inter connected with Mixpanel.

I've used this package for Flutter segment. I am adding the identify events when the user completes the signup process.

analytics?.identify(
      userId: session?.id.toString(),
      userTraits: UserTraits(
        custom: traits,
      ),
    );

before the signup I tracked some of the events that did not have user_id as value and these events keep anonymousId that is called before the identify() whereas in Mixpanel all these events should merge into single user_id that have same deviceId.

SegmentHelper.track(
        SegmentHelper.ACCOUNT_CREATED,
        properties: {
          'username': userNameController.text,
          'email': emailController.text,
        },
      );

Kindly help me how can I merge the events before signup and after signup for same user.