how to correctly use POSTMAN with PHP POST variables?

I have the following url

link

has the following code

<?php
$username  =  $_POST['username'];

$response["success"] = 1;
$response["message"] = "2";
$response["username"] = $username;

echo json_encode($response);
?>

and he returns this to me

{"success":1,"message":"2","username":null}

I am trying to use POSTMAN to be able to send the variable POST username

but without result.

example 1

example 2

Any comments?