class AddBlockForm

class AddBlockForm extends ConfigureBlockFormBase {
    use LayoutBuilderHighlightTrait;
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'layout_builder_add_block';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function submitLabel() {
        return $this->t('Add block');
    }
    
    /**
     * Builds the form for the block.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
     *   The section storage being configured.
     * @param int $delta
     *   The delta of the section.
     * @param string $region
     *   The region of the block.
     * @param string|null $plugin_id
     *   The plugin ID of the block to add.
     *
     * @return array
     *   The form array.
     */
    public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $plugin_id = NULL) {
        // Only generate a new component once per form submission.
        if (!($component = $form_state->get('layout_builder__component'))) {
            $component = new SectionComponent($this->uuidGenerator
                ->generate(), $region, [
                'id' => $plugin_id,
            ]);
            $section_storage->getSection($delta)
                ->appendComponent($component);
            $form_state->set('layout_builder__component', $component);
        }
        $form['#attributes']['data-layout-builder-target-highlight-id'] = $this->blockAddHighlightId($delta, $region);
        return $this->doBuildForm($form, $form_state, $section_storage, $delta, $component);
    }

}