Create taxonomy terms from content type field
You can trigger this behavior by implementing the hook_entity_presave on a custom module. Inside of this hook, you will call a function which creates each time the taxonomy term using the title of the node created. Here is my example:

File: test.module

use Drupal\Core\Entity\EntityInterface;

/**
 * Implements hook_entity_presave().
 */
function test_entity_presave(EntityInterface $entity) {
   if ($entity->bundle() == 'product') {
    if ($entity->isNew()) {
      $title = $entity->get('title')->getValue();
      \Drupal\test\Test::createTerm($title[0]['value']);
    }
  }
}

Assuming you have defined the createTerm function into Class Test:

File Test.php(Include this file in src folder)

namespace Drupal\test;

/**
 * Test class.
 */
class Test {
  public static function createTerm($title) {
    $term = [
      'name'     => $title,
      'vid'      => 'widget_type',
      'langcode' => 'en',
    ];
    $term = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->create($term);
    $term->save();
  }
}