Using page field value in a metatag in
You need to attach the tag in hook_node_view() (which is derived from hook_ENTITY_TYPE_view(), when the view mode is full (which makes the assumption that the full view mode is only ever used on node pages).

You can do that as follows:

function hook_node_view(array &amp;$build, \Drupal\Core\Entity\EntityInterface $mpde, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode)
{
  // Check if the view mode is full
  if($view_mode == 'full')
  {
    // Create the tag. You can set the content of the
    // metatag from the node
    $tag = [
      '#tag' =&gt; 'meta',
      '#attributes' =&gt; [
        'name' =&gt; 'name attribute of metatag',
        // Note that 'value' will differ depending on the
        // field type.
        'content' =&gt; $node-&gt;get('field_something')-&gt;value,
      ],
    ];

    // Attach the meta tag to the header.
    $build['#attached']['html_head'][] = [$tag, 'some_key_to_represent_tag'];
  }
}