Add the following code inside a function where you want to create new terms :
<?php
use Drupal\taxonomy\Entity\Term;
/**
* @param $variables
*/
function mymodule_preprocess_page(&$variables) {
$new_term = Term::create([
'vid' => 'tags',
'name' => 'Example term name',
]);
$new_term->enforceIsNew();
$new_term->save();
}
