BlockAddController.php

<?php

namespace Drupal\block\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Controller for building the block instance add form.
 */
class BlockAddController extends ControllerBase {
    
    /**
     * Build the block instance add form.
     *
     * @param string $plugin_id
     *   The plugin ID for the block instance.
     * @param string $theme
     *   The name of the theme for the block instance.
     *
     * @return array
     *   The block instance edit form.
     */
    public function blockAddConfigureForm($plugin_id, $theme) {
        // Create a block entity.
        $entity = $this->entityTypeManager()
            ->getStorage('block')
            ->create([
            'plugin' => $plugin_id,
            'theme' => $theme,
        ]);
        return $this->entityFormBuilder()
            ->getForm($entity);
    }

}