$email = [
    'from' => [
        'name' => 'My awesome startup',
        'email' => 'info@example.com',
    ],
    'recipients' => [
        [
            'name' => 'Someone',
            'email' => 'someone@example.com',
        ]
    ],
    'content' => [
        'subject' => 'Sample email',
        'text_body' => 'Plain text body',
        'html_body' => '<p>This is the <b>HTML</b> body</p>',
    ],
];
$options = array(
  'http' => array(
    'header'  => "Content-type: application/json\r\nX-Api-Key: YOUR-API-KEY",
    'method'  => 'POST',
    'content' => json_encode($email)
  )
);
$context  = stream_context_create($options);
$resp = file_get_contents('https://api.simplemailservice.eu/v1/email/send', FALSE, $context);
var_export($resp, TRUE);