Bad state: Cannot set the body fields of a Request with content-type "application/json"

You need to wrap the body in jsonEncode.

import 'package:http/http.dart' as http;
import 'dart:convert';

Map<String,String> headers = {'Content-Type':'application/json','authorization':'Basic c3R1ZHlkb3RlOnN0dWR5ZG90ZTEyMw=='};
final msg = jsonEncode({"grant_type":"password","username":"******","password":"*****","scope":"offline_access"});

var response = await post(Urls.getToken,
               headers: headers,
               body: msg,
            );