check if a data exist before POST method in Flutter?

I'm sending a POST method using http in Flutter. but before i send/ POST i need to check if the data i'm sending already exist. so how can i achieve this kind of operation.

my Model for Users.

import 'dart:convert';

List<User> usersFromJson(String str) =>
    List<User>.from(json.decode(str).map((json) => User.fromJson(json)));
String usersToJson(List<User> data) =>
    json.encode(List<dynamic>.from(data.map((e) => e.toJson())));

class User {
  int? id;
  String? name;
  String? username;
  String? email;
  String? role;

  User({this.id, this.name, this.username, this.email, this.role});

  @override
  toString() => 'User: $name';

  factory User.fromJson(Map<String, dynamic> json) => User(
      email: json['email'],
      name: json['name'],
      id: json['id'],
      username: json['username'],
      role: json['role']);

  Map<String, dynamic> toJson() =>
      {"name": name, "username": username, "email": email, "role": role};
}

my Request

Future createUser(String name, String username, String email, String password,
    String roles) async {
  final UserController userController = Get.put(UserController());
  Uri url = Uri.parse("${BASE_URL}user");
  final response = await http
      .post(url,
          headers: <String, String>{
            'Content-Type': 'application/json; charset=UTF-8',
            'Authorization': 'Bearer $TOKEN',
          },
          body: jsonEncode(<String, dynamic>{
            'name': name,
            'email': email,
            'username': username,
            'password': password,
            'roles': roles
          }))
      .then((value) => {
            if (json.decode(value.body)['error'] == null)
              {
                userController.setuserPress(true),
                Get.snackbar("created $name", "${value.body}",
                    duration: const Duration(seconds: 3),
                    colorText: Colors.black,
                    backgroundColor: Colors.white),
              }
            else
              {
                throw Get.snackbar("failed $name", "${value.body}",
                    duration: const Duration(seconds: 3),
                    colorText: Colors.black,
                    backgroundColor: Colors.white),
              }
          });
}

i'm using a form and validating it. so do i need to handle this in the form? or directly in the request?.. if so How can i achieve that?