Metatag migration from html Drupal 8.8
Modules: Metatags &amp; Migration plus. 

I am trying to convert a tag in an XML file to an open graph image metatag. 

File: 
modules/custom/my_module/dataxml.html
This is the url that has to be converted to the image metatag.

&lt;ImageUrl&gt;
  &lt;![CDATA[https://via.placeholder.com/300.jpg/09f/fff]]&gt;
&lt;/ImageUrl&gt;


File: modules/custom/my_module/config/install/migrate_plus.migration.content.yml
These are the source field for the required metatag. 

source:
  fields:
     - name: og_image_url
       label: Image
       selector: ImageUrl


Process for the required metatag

process:
  field_meta_tags:
    source: og_image_url
    plugin: prepare_meta


File: modules/custom/my_module/src/Plugin/migrate/process/PrepareMeta.php
The plugin links the metatag with the required value and serializes the mapped result

&lt;?php

namespace Drupal\my_module\Plugin\migrate\process;

use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * Plugin to prepare the meta tags.
 *
 * @MigrateProcessPlugin(
 *   id = "prepare_meta",
 * )
 */
class PrepareMeta extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $metas = [];

    // This should likely work with an array later on, but for now, it'll do.
    if (!empty($value)) {
      $metas[$this-&gt;configuration['source']] = $value;
    }

    return $metas ? serialize($metas) : [];
  }
}


When I run xDebug $metas has the following value:
$metas = [
og_image_url => "https://via.placeholder.com/300.jpg/09f/fff"
];

The node is created and the other field are migrated perfectly. However the  metatag field og:image:url remains empty and doesn't trigger an error. I don't really know what I'm doing wrong here. If someone could point me in the right direction, that would be much appreciated.
Turns out I used the wrong field name. My field is called field_meta_tag instead of field_meta_tags as used in the question above.