How to make an MVC API request from Flutter

I am building a Flutter app with user authentication. It works fine with firebase. However, I would like to code it agains SQL Server database.

I am now trying to get a user token returned from an MVC application. This is the code in my Controller:

Public Function AuthenticateUsers(ByVal APIKey As String, ByVal Password As String, ByVal ReturnSecureToken As Boolean) As JsonResult

        Dim Users As Object

        If String.IsNullOrEmpty(APIKey) Or String.IsNullOrEmpty(Password) Then
            Users = "missing parameters"
        Else
            If APIKey = "mambo" AndAlso Password = "mambo@email.com" Then
                Users = GetUsers()
            Else
                Users = "Invalid credentials"
            End If
        End If

        Return Json(Users, JsonRequestBehavior.AllowGet)

    End Function

    Private Function GetUsers() As List(Of UserModel)

        Dim usersList = New List(Of UserModel) From {
            New UserModel With {
                .idToken = 200,
                .localId = "localId",
                .expiresIn = "3600"
            }
        }

        Return usersList

    End Function

And this is the code section responsible for authentication in flutter app:

  Future<void> _authenticate(
      String email, String password, String urlSegment) async {
    // final url =
    //     'https://www.googleapis.com/identitytoolkit/v3/relyingparty/$urlSegment?key=LONGKEY';

    final url = 'https://localhost:12345/Member/AutheticateUsers';
    try {
      final response = await http.post(
        url,
        body: json.encode(
          {
            'APIKey': email.trim(),
            'Password': password.trim(),
            'ReturnSecureToken': true,
          },
        ),
      );
      final responseData = json.decode(response.body);
      if (responseData['error'] != null) {
        throw HttpException(responseData['error']['message']);
      }
      _token = responseData['idToken'];
      _userId = responseData['localId'];
      _expiryDate = DateTime.now().add(
        Duration(
          seconds: int.parse(
            responseData['expiresIn'],
          ),
        ),
      );
      _autoLogout();
      notifyListeners();
      final prefs = await SharedPreferences.getInstance();
      final userData = json.encode(
        {
          'token': _token,
          'userId': _userId,
          'expiryDate': _expiryDate.toIso8601String(),
        },
      );
      prefs.setString('userData', userData);
    } catch (error) {
      print(error);
      throw error;
    }
   }

I have commented out that line that would be successfully authenticating to firebase and rewrote a url to redirect to my MVC application.

So now the problem is I am getting the error that the request is refused. It is the first time I am trying to get external application call an API. So I think I could be missing some prerequisites/formats.

Could someone please guide me on how to make this work.

Thanks!