How do I create function to pass Json data in Flutter?

How do I create a function to call JSON data on a specific page? The value of the JSON data will be used to be put in the TextField.

I have created the model class and the API for the User data. I want to use the id and the code.

user.dart

import 'dart:convert';

List<User> userFromJson(String str) => List<User>.from(json.decode(str).map((x) => User.fromJson(x)));

String userToJson(List<User> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class User {
    User({
           ....

user-api.dart

import '../model/user.dart';
import 'package:http/http.dart' as http;
import 'package:shared_preferences/shared_preferences.dart';

Future<List<User>> fetchUser() async{
  SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
  var loginID = sharedPreferences.getString('user');
  var url = Uri.parse("https:wahatever-abc/user.php?LoginID=$loginID");
  final response = await http.get(url);
  return userFromJson(response.body);
}

I want to create a function to call the JSON data in a new page call new_user.dart. The data in the function will be used in the Textfield form. I have no idea on how to call it.

new_user.dart

import 'model/user.dart';
import 'api/user-api.dart';
import 'package:http/http.dart' as http;

 @override
  void initState() {
    fetchUserA();
    super.initState();
    _myActivity = '';
    _myActivityResult = '';
  }

   
   void fetchUserA() async {
     
     ....
   }

How do I create and pass the value inside the fetchUser?

What I want to achieve is how do I call the value from the JSON and declare it in the fetchUserA function by using the 'fetchUser()' in the 'user-api.dart' With the called value in the fetchUserA i will use the data to insert it in the Textfield form which i will create it later.

example of what i want to try

This is what i tried before but its not working. So I wanted to try other way. What I mean by other way is try to do like this this example

 Widget build(BuildContext context){
    return Form(
      key: _formKey,
      autovalidateMode: AutovalidateMode.always,
      child: FutureBuilder(
         future: fetchUser(),  
          builder: (context, snapshot){
             return ListView.builder(
              itemCount: snapshot.data.length,
              shrinkWrap: true,
              itemBuilder: (BuildContext context, index)
              {
                User user = snapshot.data[index];
                return Padding(
                   padding: const EdgeInsets.all(8.0),
                 child: Column(  
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    Row(
                      crossAxisAlignment: CrossAxisAlignment.start,
                      children: <Widget>[
                        Expanded(
                          child: Padding(
                            padding: const EdgeInsets.all(8.0),
                          
                            child: TextFormField(
                              
                                decoration : InputDecoration(labelText: "${user.id}"),
                                
                                controller: id,
                            ),
                            ),
                          ),