Use FutureBuilder

you can check the link for more details about FutureBuilder

edit your register function to return data then use FutureBuilder to view list of data inside build

Future  register(String nom,String prenoms,String mail, telephone,naissance, pseudo, passe1, 
     passe2
    ) async {
      setState(() {
        msg='';
      });
    try {
      
      var url = Uri.parse('https://konamicash.com/register_app');
      var response = await http.post(url, headers: {
        "Accept": "application/json",
        "Access-Control-Allow-Origin": "*"
      },body: {
        "nom_utilisateur":nom,
        "confirmation_du_mot_de_passe":passe2,
      });var codes= response.statusCode;
   

       if(response.statusCode == 200) {
     
       return  json.decode(response.body);
       

      } else {
        print('Error: ${response.statusCode}');
      }
    } catch (e) {
      print('An error occurred: $e');
    }
   }

i try to fix you code.

In condition we already know the response will be Map:

{'status':0, 'message': 'success'}

I already commented what i add and the use of the code.

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

class StfNouveauCompte extends StatefulWidget {
  const StfNouveauCompte({super.key});

  @override
  State<StfNouveauCompte> createState() => _StfNouveauCompteState();
}

class _StfNouveauCompteState extends State<StfNouveauCompte> {
  String msg = "";
  var jsonList;
  final _formKey = GlobalKey<FormState>();
  final controllernom = TextEditingController();
  final controllernconfirmationmotdepasse = TextEditingController();

  // New variable to save the data from API.
  //
  // [dataFromApi] is nullabel since we need to wait the response from API.
  Map<String, dynamic>? dataFromApi;

  @override
  void dispose() {
    super.dispose();
    controllernom.dispose();
    controllernconfirmationmotdepasse.dispose();
  }

  String getRegisterStatus() {
    // If dataFromApi is null or empty return 'none'
    if (dataFromApi == null || dataFromApi!.isEmpty) {
      return 'None';
    } else {
      // If [dataFromApi] is already assigned by response data then returning
      // the status from response data.
      return '${dataFromApi?['message']}';
    }
  }

  Future<void> register(String nom, String prenoms, String mail, telephone,
      naissance, pseudo, passe1, passe2) async {
    setState(() {
      msg = '';
    });
    try {
      var url = Uri.parse('https://konamicash.com/register_app');
      var response = await http.post(url, headers: {
        "Accept": "application/json",
        "Access-Control-Allow-Origin": "*"
      }, body: {
        "nom_utilisateur": nom,
        "confirmation_du_mot_de_passe": passe2,
      });

      var codes = response.statusCode;

      if (codes == 200) {
        // Show response in console log
        print('[INFO] your response:n$dataFromApi');

        // assign response data to [dataFromApi]
        dataFromApi = json.decode(response.body);

        // Rebuild state to inform state that [dataFromApi] is changed it's value
        setState(() {});
      } else {
        print('Error: ${response.statusCode}');
      }
    } catch (e) {
      print('An error occurred: $e');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Namis.com'),
        backgroundColor: Colors.pink,
      ),
      body: Center(
        child: SingleChildScrollView(
          child: Form(
            key: _formKey,
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.stretch,
              children: [
                Container(
                  margin: EdgeInsets.only(bottom: 15),
                  child: Text(
                    'Register',
                    style: TextStyle(
                        fontSize: 35,
                        fontWeight: FontWeight.bold,
                        color: Colors.grey.shade400),
                    textAlign: TextAlign.center,
                  ),
                ),
                Container(
                  padding: EdgeInsets.all(20),
                  margin: EdgeInsets.only(bottom: 10),
                  child: TextFormField(
                    keyboardType: TextInputType.text,
                    decoration: InputDecoration(
                      labelText: 'Nom',
                      border: OutlineInputBorder(),
                      suffixIcon: Align(
                        widthFactor: 1.0,
                        heightFactor: 1.0,
                        child: Icon(
                          Icons.person,
                        ),
                      ),
                    ),
                  ),
                ),

                Container(
                  padding: EdgeInsets.all(20),
                  margin: EdgeInsets.only(bottom: 10),
                  child: TextFormField(
                    obscureText: true,
                    keyboardType: TextInputType.text,
                    decoration: InputDecoration(
                      labelText: 'password',
                      border: OutlineInputBorder(),
                    ),
                    controller: controllernconfirmationmotdepasse,
                  ),
                ),
                SizedBox(
                  width: double.infinity,
                  height: 50,
                  child: ElevatedButton(
                    onPressed: () {
                      if (_formKey.currentState!.validate()) {
                        final nom = controllernom.text;

                        final confirmationmotdepasse =
                            controllernconfirmationmotdepasse.text;

                        FocusScope.of(context).requestFocus(FocusNode());

                        ScaffoldMessenger.of(context).showSnackBar(
                            const SnackBar(content: Text('Envoi en cours')));

                        register(
                            nom,
                            prenoms,
                            adressemail,
                            telephone,
                            datedenaissance,
                            pseudo,
                            motdepasse,
                            confirmationmotdepasse);
                      }
                    },
                    child: Text('Créer le compte'),
                  ),
                ),

                // New widget to show your response data
                Center(
                  child: Padding(
                    padding: const EdgeInsets.all(16),
                    child: Text('Register status = ${getRegisterStatus()}'),
                  ),
                ),

                Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Text("J'ai déjà un compte..."),
                    TextButton(
                      onPressed: () {},
                      child: Text(
                        'Je me connecte',
                        style:
                            TextStyle(color: Color.fromARGB(255, 245, 11, 11)),
                      ),
                    ),
                  ],
                ),
                //just bellow the sized box where i want to display my different message result_message
                SizedBox(
                  height: 30,
                ),
                Text(
                  msg,
                  style: TextStyle(color: Colors.red),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

This is just the basic example to use the data for the widget. I suggest you to learn FutureBuilder for data fetching from API.

Hope it helps! If you have any question please just ask:) thanks.