How to alter a layout_paragraphs paragraph edit form

Altering a Layout Paragraphs paragraph create / edit form sounds easy and it indeed is, but you have to know, which hook to use.

To make it easier for you, here's how:

Use hook_form_layout_paragraphs_component_form_alter() which can be found in layout_paragraphs.api.php.

Here's an example implementation adding links into the 'block_content' paragraphs bundle:

/**
 * Implements hook_form_layout_paragraphs_component_form_alter().
 */
function drowl_paragraphs_type_block_content_form_layout_paragraphs_component_form_alter(array &$form, \Drupal\Core\Form\FormStateInterface &$form_state) {
  if (!empty($form['#paragraph']) && $form['#paragraph']->bundle() == 'block_content') {
    $form['block_content_library'] = [
      '#title' => t('Custom block library'),
      '#type' => 'link',
      '#attributes' => [
        'class' => ['button', 'button--primary', 'button--small', 'button--block-content-library'],
        'target' => '_blank',
      ],
      '#url' => \Drupal\Core\Url::fromRoute('entity.block_content.collection'),
    ];
    $form['block_content_add_page'] = [
      '#title' => t('Add custom block'),
      '#type' => 'link',
      '#attributes' => [
        'class' => ['button', 'button--action', 'button--small', 'button--block-content-add-page'],
        'target' => '_blank',
      ],
      '#url' => \Drupal\Core\Url::fromRoute('block_content.add_page'),
    ];
  }
}

Hopefully you won't need to search for it as long as I had to. ;)

This is the related issue: https://www.drupal.org/project/drowl_paragraphs/issues/3347228