Set taxonomy field on node creation
thinking the fastest way to do this one is hook_node_insert() ...
see https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21entity.api.php/function/hook_ENTITY_TYPE_insert/8.2.x

if i am following what your saying, you want to set the value of 'field_section' the newly created taxonomy term in tags? if so the following will get you started .. 

/**
 * Implements hook_ENTITY_TYPE_insert().
 */
function YOURMODULE_node_insert(EntityInterface $entity) {
  $type = $entity-&gt;bundle();
  if (!empty($type) &amp;&amp; $type == 'blog_entry') {
      $field_tags_terms = $entity-&gt;field_tags-&gt;referencedEntities();
      // Your selected term id.
      $field_section_id = $entity-&gt;field_section-&gt;target_id;
      if (!empty($field_tags_terms) &amp;&amp; !empty($field_section_id)) {
          foreach ($field_tags_terms as $term) {
              $term-&gt;set('field_section', $field_section_id);
              // or 
              // $term-&gt;field_section-&gt;appendItem(['target_id' =&gt; $field_section_id]);
              $term-&gt;save();
          }
      }
  }