<?php
    $objectManager = MagentoFrameworkAppObjectManager::getInstance();
    $product = $objectManager->get('MagentoFrameworkRegistry')->registry('current_product');//get current product
?>

<!--get product sku-->      <?php $current_sku = $product->getSku(); ?>
<div class="field required">
                    <label class="label" for="amfaq-title">
                        <?= $block->escapeHtml(__('Your question:')); ?>
                    </label>
                    <div class="control">
                        <textarea name="title"
                                  id="amfaq-title"
                                  title="<?= $block->escapeHtml(__('Question')); ?>"
                                  class="required-entry input-text"
                                  cols="5"
                                  rows="3"
                                  data-bind="value: title"
                                  placeholder="<?= $block->escapeHtml(__('Type your question here ...')) .'('. $current_sku .')' ;?>"></textarea>
                    </div>
                </div>