HttpClient.PostAsync and PHP api

Hello All respected SO members,

I am trying to build an android app with .NET MAUI.
I tried to save a content to MySQL database using HttpClient.PostAsync through PHP Api, however I can not make it work (the response is Success but the data is not saved to the table), while when I tried using POSTMAN to test with the PHP Api, everything is ok and it is able to save into the desired table.

.NET MAUI code

EmployeeData emp = new EmployeeData();
emp.emp_pin= EmpID.Text;

HttpClient cl = new HttpClient();
string js = JsonSerializer.Serialize(emp);
StringContent httpContent = new StringContent(js, Encoding.UTF8, "application/json");
var response = cl.PostAsync(Url, httpContent).Result;
if (response.IsSuccessStatusCode)
{
    DisplayAlert("Success", "Success Record Your Data.", "OK");
}
else
{
    DisplayAlert("Fail", "Please Try Again.", "OK");
}

Model

public class EmployeeData
{
    public string emp_pin{ get; set; }
}

PHP API

<?php
require_once('dbhelper.php');

$json = file_get_contents('php://input');
$data = json_decode($json, true);

$emppin=$data['emp_pin'];
$sql = $conn->prepare("INSERT INTO test (emp_pin) VALUES (?)");
$sql->bind_param('s', $emppin);
$sql->execute();
if ($sql) {
    echo json_encode(array('RESPONSE' => 'SUCCESS'));
} else {
    echo json_encode(array('RESPONSE' => 'FAILED'));
}

Using POSTMAN, the data is successfully saved to the table.
enter image description here

using the JSON Visualizer,
enter image description here

Please highlight to me which part I made a mistake on.