OneSignal - Send scheduled notification

I want to send scheduled messages for OneSignal but instead of using their dashboard I want to use an API to do this and I did read their documentation but I could not find anything is there away to modify their current API so I can send the scheduled notification

function sendMessage(){
        $content = array(
            "en" => $message,

            );

    $fields = array(
        'app_id' => "XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX",
        'included_segments' => array('All'),
  'data' => array("foo" => "bar"),
        'contents' => $content
    );

    $fields = json_encode($fields);
print("\nJSON sent:\n");
print($fields);

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, "https://onesignal.com/api/v1/notifications");
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json; charset=utf-8',
                                               'Authorization: Basic NGEwMGZmMjItY2NkNy0xMWUzLTk5ZDUtMDAwYzI5NDBlNjJj'));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    curl_setopt($ch, CURLOPT_HEADER, FALSE);
    curl_setopt($ch, CURLOPT_POST, TRUE);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

    $response = curl_exec($ch);
    curl_close($ch);

    return $response;
}

$response = sendMessage();
$return["allresponses"] = $response;
$return = json_encode( $return);



print("\n\nJSON received:\n");
    print($return);
  print("\n");