How can I make an http POST request from dart with authentication?

Question

I'm trying to do a post, to create a user, but in a simple way. I do need to use authentication, so I'm passing the token in the header, as I would do in the GET request. But I'm not sure why I'm receiving the error "400 - Bad Request".

import 'package:http/http.dart';

{...}

Future test() async {
    Response response = await post('https://access.altaviu.com/Data/api/Account/Register',
        body: jsonEncode({
          "Email": "useruser@gmail.com",
          "Password": "QWERTY1234!",
          "ConfirmPassword": "QWERTY1234!",
        }),
        headers: {
          'Content-Type': 'application/json',
          'Accept': 'application/json',
          'Authorization': 'Bearer ${UserAccountData.token}',
        });

    if (response.statusCode == 200) {
      print('SUCCESSFUL TEST');
    } else {
      print('ERROR IN THE TEST ${response.statusCode} - ${response.reasonPhrase}');
    }
  }

I'm using the library (or package) "http: 0.12.0+2"