output.md

This is the output when the field has a value:

juampy@Juampy/var/www/drupal8(8.0.x)$ drush scr script
string(32) "Some text Un articulo more text."

This is the output when the field does not have a value:

juampy@Juampy/var/www/drupal8(8.0.x)$ drush scr script
string(38) "Some text [node:field_slug] more text."

script.php

<?php

use Drupal\node\Entity\Node;

$node = Node::load(102);
$text = 'Some text [node:field_slug] more text.';
$token_service = \Drupal::service('token');
$processed_text = $token_service->replace($text, ['node' => $node]);
var_dump($processed_text);