Programmatically assign meta tags to an entity

metatag_entity.php

// @see https://www.drupal.org/docs/8/modules/metatag

$entity_type = 'node';
$values = [
  'nid' => NULL,
  'type' => 'article',
  'title' => 'Testing metatag creation',
  'uid' => 1,
  'status' => TRUE,
  'field_meta_tags' => serialize([
    'title' => 'Some title',
    'description' => 'Some description.',
    'keywords' => 'Some,Keywords',
  ]),
];
$node = \Drupal::entityTypeManager()->getStorage($entity_type)->create($values);
$node->save();