Get correct ISO DateTime String with offset in Flutter: https://dartpad.dev/84d855e41c0134a34ff8b2cf034ad249

date_utils.dart

import 'package:intl/intl.dart';
import 'package:meta/meta.dart';

void main() {
  print(DateUtils.formatISOTime(DateTime.now()));

  print(DateUtils.getCurrentISOTimeString());
}

class DateUtils {
 
 ///Converts DateTime to ISO format 
 ///Output: 2020-09-16T20:41:09.331+05:30
  static String formatISOTime(DateTime date) {
    var duration = date.timeZoneOffset;
    if (duration.isNegative)
      return (date.toIso8601String() +
          "-${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
    else
      return (date.toIso8601String() +
          "+${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
  }

 ///get ISO time String from DateTime
 ///Output: 2020-09-16T20:42:38.629+05:30
  static String getCurrentISOTimeString({DateTime dateTime}) {
    var date = dateTime ?? DateTime.now();
    //Time zone may be null in dateTime hence get timezone by  datetime
    var duration = DateTime.now().timeZoneOffset;
    if (duration.isNegative)
      //TODO: convert duration to abs value instead of below params
      return (date.toIso8601String() +
          "-${duration.inHours.abs().toString().padLeft(2, '0')}:${(duration.inMinutes.abs() - (duration.inHours.abs() * 60)).toString().padLeft(2, '0')}");
    else
      return (date.toIso8601String() +
          "+${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
  }
}

Thank you for sharing.
I did few changes on it:

class DateUtils {
 
 ///Converts DateTime to ISO format 
 ///Output: 2020-09-16T20:41:09.331+05:30
  static String formatISOTime(DateTime date) {
    var duration = date.timeZoneOffset;
    
    return "${date.toIso8601String().replaceAll('Z', '')}${duration.isNegative ? "-" : "+"}${_formatTimeZoneOffsetISO(duration)}";
  }
  
  static String _formatTimeZoneOffsetISO(Duration duration) {
    duration = duration.abs();
    return "${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}";
  }

 ///get ISO time String from DateTime
 ///Output: 2020-09-16T20:42:38.629+05:30
  static String getCurrentISOTimeString({DateTime? dateTime}) {
    return formatISOTime(DateTime.now());
  }
}

You have a big bug there, please remove the negative logic.
If you are in a timezone that is negative you will have double minus

  /// converts [date] into into string format
  /// Example:
  /// * Positive:   (UK)   `2020-09-16T11:55:01.802248+00:00`
  /// * Negative: (Canada) `2020-09-16T11:55:01.802248-08:00`
  static String formatISOTime(DateTime date) {
    var duration = date.timeZoneOffset;
    /// If the user is in Canada the time zone is GMT-8 then the signal will need to be negative.
    /// Because we already get the minus from the hours in the string then we don't need to add it to the string.
    /// In the case the timezone is GMT-0 or higher then the sign will need to be positive.
    var timezoneSignal = !(duration.isNegative) ? '+' : '';
    var dateString = (date.toIso8601String() + "${timezoneSignal}${duration.inHours.toString().padLeft(2, '0')}:${(duration.inMinutes - (duration.inHours * 60)).toString().padLeft(2, '0')}");
    return dateString;
  }

@loonix, the code will not have a double minus if it's using the absolute value of the duration (.abs()), but your suggestion would also work.