The send function does not work in a custom form
You have to write the submit handler for the form to save data.

  /**
   * {@inheritDoc}
   */
  public function submitForm(array &amp;$form, FormStateInterface $form_state) {

    // Insert queries.

    $query = \Drupal::database()-&gt;insert('mymodule_table');
    $query-&gt;fields([
      'name' =&gt; $form_state-&gt;getValue('name'),
      'tel' =&gt; $form_state-&gt;getValue('tel'),
    ]);
    $query-&gt;execute();

  }